Order of operations (BODMAS) $$40-20/2+15\times1.5\\\hspace{.1in}\\40-20/2+15\times1.5=\\ 40-10+22.5=7.5$$
I'm studying and this is from an example. 
In BODMAS, aren't addition and subtraction have same level?   
So, in the 3rd line, it should be from left to right, correct?
 A: $$40-10+22.5=52.5$$
You're correct, subtraction and addition have the same level of priority, so when both exist, then you operate from left to right. You are entirely correct about that.  If you were puzzled when you saw the "answer" $\,7.5$, good for you, since $\;\;40-10+22.5\neq 7.5!$
Now, note: $$40-(10+22.5)=7.5$$
because operations in "brackets" (or parentheses) have greatest priority.
So let's be forgiving: perhaps the author of the problem statement/review notes forgot parentheses!
A: I think you have your answer but still I want to clear this thing to you.
$$40-10+22.5$$
 you are writing it as    $40-(10+22.5)$ but if you try to remove this you will get $40-10-22.5$ which is not same as previous line so if you want to do add and subtraction in a line (they have same priority) you can think as $10$ and $22.5$ have opposite sign so we write their difference and put the sign of bigger number to the difference so here we get
$-10+22.5=12.5$
then we solve this 
$40+12.5=52.5$
A: The example is incorrect under BODMAS, but typically, people use a different set of rules, to the effect of creating implied brackets.


*

*Brackets unchanged

*Of (ie functions) unchanged.

*M  Multiplication of close-fit numbers.

*D  If there is a horizontal fraction, then this is evaluated first.  For example, $/$ only is evaluated here. 

*MD  Multiplication and division are evaluated equally when set with equal-height signs.  This is a different division $\div$.  So $120/2 \div 60/2=2$ means 60/30, but $120 \div 2 \div 60 \div 2=0.5$.  This is a product of numbers, where $\div$ is read as a unitary inverse.  Rather like AS, really.

*AS  addition and subtraction are equally evaluated, as if the sign were attached to the number, when set with equal-height signs.  eg 5-2+3 = 6


Here is an example of how this works.
$F = c/4\pi þ^2 * M^2/R^2$
In this equation, the multiplication represented by * is a lower operation (MD) then the multiplications represented by {4\pi þ^2}.  That is we see this as this, with the close-fit or inner multiplication, being developed done before the division /.  In turn, once these are evaluated, the product occurs from left to right in the manner of computers.
$F = (c/(4\pi þ^2)) * (M^2/R^2) = c/4/\pi \ þ^2 * M^2 / R^2$
A: $\boxed{40-20/2+15\times1.5}$  
Using brackets we can remove ambiguity:
$40+(-20/2)+(15\times1.5)$
$=40+(-10)+(22.5)$
$=(40+22.5)+(-10)$
$=62.5-10$
$=52.5$
A: You can type this BODMAS mathematical expressions in http://www.careerbless.com/calculators/ScientificCalculator/ and click on solve and see how these are processed
A: As @GerryMyerson noted, there was indeed an error. The last two terms were grouped, which changed the expression to $40 - (10 + 22.5)$, which is entirely different from the correct expression. This is equivalent to $40 + (-1)(10 + 22.5)$, or $40 - 10 - 22.5$. Note how the second plus sign has become a minus sign.
