I've got a solution to a economy class calculation, but I can't figure out how they came to the answer. Could you help me?

-5p + 80 = 8p +-50

.......? = ......?

120 = 12p

p = 10

Extra: do you know which rule they are using to get from the first line to the 3d?

Many thx

Math beginner, Frank

  • $\begingroup$ I am glad if you understood this, the same idea/technique with perhaps a bit more complexity is frequently used to solve several kinds of mathematical problems. $\endgroup$ Commented Nov 13, 2010 at 20:41

2 Answers 2






130=13p (to get 120=12p at this step, you can multiply both sides by 12/13, but it's not necessary to arrive at the final answer)


  • $\begingroup$ thank you! cause of confusion - 1 @%$ multiplication error $\endgroup$
    – Frank
    Commented Nov 13, 2010 at 20:21

Add $5p+50$ to both sides of the = sign, and then simplify each side. (If you wish you can do this in two steps: Add $5p$ and then 50 to each side of the equality sign.)

EDIT: The above leads to the equation $130 = 13p$ which can be solved directly by dividing both sides of the equality sign by 13: $130/13 = 13p/13$, which after simplification gives us $10=p$ -- which is the same as saying $p=10$.

Note: To reach the equation $120=12p$, that is contained in the original question, is not necessary in order to solve the equation.

  • $\begingroup$ still don't get it - after the start I just end up with: 120 = 13p instead of 120 = 12p ..? what am I missing? $\endgroup$
    – Frank
    Commented Nov 13, 2010 at 15:04
  • 1
    $\begingroup$ You are missing that 80+50=130, not 120. $\endgroup$ Commented Nov 13, 2010 at 15:40
  • $\begingroup$ many thanks - my bad for the multiplication error $\endgroup$
    – Frank
    Commented Nov 13, 2010 at 20:20
  • $\begingroup$ @Ross Millikan: You are right, fixed it. $\endgroup$ Commented Nov 14, 2010 at 5:15

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