How to Calculate Percent Increase and Decrease on the ACT
Many students find math questions that involve percent increase and percent decrease somewhat confusing. The first step to answering these questions correctly on the ACT is identifying them when they’re presented.
Some common scenarios for percent increase questions are

Sales tax added to the price of an item

Tipping a server at a restaurant

Interest earned on an investment
Some typical situations for percent decrease questions are

Money lost on an investment

Discount on an item being sold

Deduction from a paycheck due to taxes
After you know whether you’re dealing with percent increase or percent decrease, here’s how you handle the calculations:

Increase. Calculate a percent increase as 100 percent + the percent. For example, a percent increase of 15 percent is equal to

Decrease. Calculate a percent decrease as 100 percent – the percent. For example, calculate a percent decrease of 20 percent as
The following two examples show you how to handle both types of questions from start to finish.
Example 1
Randy bought a small guitar amplifier priced at $165 with a special coupon that gave him a 15percent discount. About how much did he end up paying for the amp?
(A) Less than $100
(B) Between $100 and $120
(C) Between $120 and $140
(D) Between $140 and $160
(E) More than $160
A 15percent discount is a percent decrease of 15 percent:
Use your calculator to find this percentage of the original price of $165:
Thus, the correct answer is Choice (D).
You can apply this method for finding percent increase and decrease to many percent problems.
Example 2
Keith’s portfolio is currently worth $10,200, representing a 20percent increase on his original investment. How much did he originally invest?
(F) $7,800
(G) $8,160
(H) $8,440
(J) $8,500
(K) $8,880
A 20percent increase is calculated as
so use the following formula:
Change the percent sign to 0.01 and solve:
Therefore, the correct answer is Choice (J).