SOLUTION: The cost of petrol increased by 15% one week but fell back to the original price the next week. By what percentage did the cost of petrol fall in the second week?

Algebra ->  Percentage-and-ratio-word-problems -> SOLUTION: The cost of petrol increased by 15% one week but fell back to the original price the next week. By what percentage did the cost of petrol fall in the second week?      Log On


   

Question 1020249: The cost of petrol increased by 15% one week but fell back to the original price the next week.
By what percentage did the cost of petrol fall in the second week?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Let's say the price starts at X.
P=X
During the 15% increase,
P=%281%2B15%2F100%29X=1.15X
During the unknown decrease,
P=%281%2BY%2F100%29%281.15X%29 "Y is the percentage decrease you're looking for"
Make that new price equal to the original price, X.
%281%2BY%2F100%29%281.15%29X=X
You can divide X out of both sides.
Now solve for Y.
1.15%2B%281.15Y%29%2F100=1
Subtract 1.15 from both sides
%281.15%29Y%2F100=-0.15
Multiply both sides by 100 and divide both sides by 1.15.
Y=-15%2F1.15
Y=-13.04
About a 13% decrease.