gas costs $2.53 per gallon in May. In September, gas cost $3.05 per gallon. What was the percent change in the cost of a gallon of gas?

First, find the difference:
$3.05 - $2.53
= $0.52
Take the positive difference and divide with the original price:
0.52 / 2.53
= 0.2055(bunch of numbers)
Then, multiply that by 100 or move the decimal 2 places to the right.
20.55 (rounded)
Your answer is:
20.6%