The Price is Right optimal play The following situation happened on the Price is Right and I was curious about the optimal response.
The rules are:
A contestant rolls a wheel with 5 cent increments from 5 - 100 (20 numbers total). A contestant can choose to spin the wheel once and accept the number the wheel landed on (stay) or spin again and have this new number added to the previous number. If the contestant has a number that is over 100 cents they automatically lose. The object of the game is to roll the highest number out of a set of contestants.
GirlA rolled once and rolled 60 cents. GirlA knows GirlB will play afterwards. Should GirlA roll again? At what number is the expected value neutral / what range of numbers should GirlA stay on? If there is more than one player behind, what range of numbers is best to stay on?
 A: Long story short: She should not roll again. 
Supposing Girl A does not spin again, we have the following possibilities. 
Case 1: It is clear that there is an $\frac{8}{20}=\frac{2}{5}=40$% chance that Girl B rolls (strictly) higher than 60 (in which case, Girl B wins). In this case, Girl B holds (and presumably wins).
Case 2: Girl B rolls less than or equal to 60. This outcome is $60$% likely. Then, Girl B must roll again to beat girl A. Note that if Girl B rolls a $5n$ on her first attempt, where $n\in \{1,...,12\}$, the probability of her rolling "over" is $5n$%. What Girl B needs to roll enough so that her score is greater than 60 (and not exceeding 100) is $\{60-5(n-1),...,100-5n\}$. This occurs $\frac{8}{20}=40$% of the time. 
Putting all of this together, if Girl A remains at 60, then Girl B has a $.4+.6\cdot.4 =.4+.24=.64=64$% chance of getting a higher score than Girl A. 
Now, if Girl A decides to roll again, she has a $60$% of rolling over (or losing right away). Given this new score, say $5m$ (that is not a roll over), we have that Girl B can either win on her first roll or roll again. Note that $m\in \{13,...,20\}$. 
Case 1: Girl B rolls greater than Girl A (in the case where $m\neq 20$) we have this is $100-5m$%.
Case 2: Girl B rolls less than or equal to than Girl A. This has probability $5m$%. Suppose that Girl B rolls $5t$. Then, the probability that Girl B, on her second turn, gets higher than Girl A, but not rolling over, we have $\{5m-5(t-1),...,100-5t\}$. 
Now, we need to put these together to arrive at our final answer: The probability of Girl A not rolling over and Girl B beating her is $(.4)(\sum_{i=1}^8(.1)(\frac{8-i}{20}))=(.4)(\frac{7}{50})=(.4).14=0.056$. Therefore, the probability of Girl A losing is $.6+0.056=.656=65.6$%. 
In conclusion, Girl A is in a very tough position. There is a $65.6$% chance of her losing if she rolls again and only a $64$% chance of losing if she chooses to not roll. Therefore, it's very close, but she shouldn't roll again.
A: I made a python function to test this problem.I will put the code at the bottom and you may test it at this site.
Please note that I used a 1 to 100 system rather than a 1 to 20 system.
Scroll to bottom for conclusion
I found that when the first number that is spun gets to 50 the odds of you winning and losing are approximately equal. 
However, as the number you first spin gets higher, the average amount you gain drops.
Here are 3 test each with 100,000 spins.(Please note these are just approximations) 
When the number you have spun on your first spin is 40.
Wins:59473
Losses:40527
Average gain:30.46

When the number you have spun on your first spin is 50.
Wins:49593
Losses:50407
Average gain:25.505

When the number you have spun on your first spin is 60.
Wins:39597
Losses:60403
Average gain:20.466727277318988

As you can see, every time the number we spun at first is increased by 10, the average amount gained goes down by 5.
The obvious conclusion is that if you rolled a 60, you should definitely not spin again.
The highest roll were  the odds would be in your favor would be a 45. 
from random import randint

def test(spin):
    w=0
    l=0
    gain=0
    wins=0
    tests=1000
    for i in range(tests):
        new=spin+randint(1,101)
        if new>100:
            l+=1
        else:
            w+=1
            gain+=new-spin
            wins+=1

    return ("Wins:"+str(w)+"\nLosses:"+str(l)+"\nAverage gain:"+str(gain/wins))



#Set This equal to the number the person spun first
first_spin=50

print(test(first_spin))

A: There is actually a published paper about that question:
"To Spin or Not To Spin? Natural and Laboratory Experiments from The Price is Right", Rafael Tenorio and Timothy N. Cason, The Economic Journal, 2002.
Your question is answered in proposition 1:

Should girlA roll again?

No.

At what number is the expected value neutral / what range of numbers
  should girlA stay on?

At 50 she is indifferent and for >50 she should stay.

If there is more than one player behind, what range of numbers is best
  to stay on?

For 3 they show that 65 is the number to stay on, for a higher number of players? No idea (and it is not really easy to derive as you have to recalculate the strategies of everyone else).
Btw: They state that Contestant 1 wins 30.82% of the time, Contestant 2 wins 32.96% of the time, and Contestant 3 wins 36.22% of the time.
A: More on this problem in the paper P.Coe and W. Butterworth, Optimal stopping in the showcase showdown, American Statistician, 49, 1995, p. 271-275.  The problem and variants are also extensively  discussed in the book Understanding Probability by Henk Tijms
A: This is a great question and I record the Price is Right daily to watch only the big wheel.  I looked at the proposed answers above and they are brilliant answers to the wrong question.  
The reason the calculation of the probabilities is so difficult is that there are three people involved, each with a choice of whether to spin the wheel once or twice, except in the instance where the first two contestants go over a dollar in which case the third contestant automatically wins, but only gets to spin the wheel once for an initial score.  The further complication is that there can be ties which does not appear to be accounted for in the calculations above. If there is a tie and the tie is the highest score without going over, there is a spin-off where each contestant gets one additional spin and the dollar bonus is still in play. Further, if any contestant's spins total one dollar, they are awarded $1,000 and an additional spin.  If on an additional spin, they land on 5 or 15 cents, they win an additional $10,000 and if they land on a dollar, they win an additional $25,000.  One more thing to consider is that the numbers are in a particular non-sequitur order.  I would say maybe that doesn't matter except that a contestant can't just spin the wheel targeting a particular range (other than the 5 cent, 1 dollar, 15 cent range on the additional spin) even though many try to do so.
The winning percentages pointed out in the article seem about right, intuitively, and while the calculations above suggest that the first person to spin is severely disadvantaged, by observation, it has not been the case. (I have been recording the show and watching who wins on the big wheel for nearly a year.  That's about 420 actual big wheel contests.) The third person already know the scores of the first two contestants and has no incentive to quit spinning with a losing number.  The second contestant knows the score of the first and again has no incentive to quit unless they either beat or tie the first contestant.  The first contestant knows that there will be two contestants spinning after them and has an incentive to try to get a higher number
But, each contestant, at least on their first spin has no "feel" for the wheel, so it might be fair to say that the first spin of each contestant is completely random.  And, the second spin may only be slightly influenced by the "feel" gained in the first spin.  Again, the order of numbers on the wheel is not random, but I don't have it to give. So, at least for the initial two spins, I would say you could ignore "feel," but it is the whole idea of the dollar bonus spin, so don't think contestants are unaware of this factor.  
All of that said, on the very first spin of each contestant, the probability for any score is going to be 1/20.  To determine if the first contestant should take a second spin, the risks are:
A.   What is the probability that the score on the first spin, when added to the score of the second spin will exceed one dollar and cause the contestant to automatically be disqualified?
B.   What is the probability that on one spin, the second contestant will a)tie or b) surpass the total of the first spin.
C.   What is the probability that on two spins, the second contestant will a)tie or b) surpass the total of the first spin without going over a dollar.
D.   What is the probability that on one spin, the third contestant will a) tie or b) surpass the total of the first spin.
E.   What is the probability that on two spins, the third contestant will a) tie or b) surpass the total of the first spin.
F.   What is the probability that the first contestant on a second spin, would score a total of one dollar or less. (This should have been first or second in my list.)
So, what I get from this list is that if:
A + F >= Ba + Bb + Ca + Cb 
the first contestant should not spin.  
If the first spin is 50 cents, then A + F would be 1.  Ba would be .5.  Bb would be .25.  Ca would be .5 and Cb would be .25. The total for the two following contestants is 1.5, the first contestant should spin again.
If the first spin is 55 cents, then A + F would be 1, so there is a flaw.  A should be the probability that the second spin will cause the total to be less than or equal to a dollar so that the term totaling to spin again number goes down as the score in the first spin goes up.  Recalculating that way, A+F equals .45 + .45, .90.  I'm missing something but will push through to see if it shakes out.  Ba would be .45, Bb would be (Okay, well first, tie would only be 1/20 on the first spin always, so that part is wrong. Wow, I am crumbling, I totally forgot D and E above.  I guess I'm too sleepy.) Help!
I broke down most of the elements of the probability problem, although something is telling me I still missed two of them (one I suspect is ties on the first contestant's side of the equation).  But, I can't think through how to set up the equation.  The rules are there.  Holy smoke, its 2:30 am.
