## Sequential Percent Changes

Summary
Understanding sequential percent changes is crucial for avoiding common mistakes on the GRE, particularly in problems involving a series of increases and decreases.
• A same percent increase followed by a decrease does not return to the original value.
• Never add or subtract percent changes directly; this approach is always incorrect.
• Use multipliers to accurately calculate the effect of sequential percent changes.
• Common mistakes include assuming that a percent up followed by the same percent down cancels out, and adding or subtracting percentages to find the net change.
• Correctly using multipliers reveals the true impact of sequential percent changes, avoiding predictable mistakes.
Chapters
00:01
The Trap of Sequential Percent Changes
00:10
Common Mistakes and How to Avoid Them
01:42
Applying Multipliers Correctly
06:16
Understanding the Impact of Sequential Changes

Q: Why is 1.3 the multiplier for a 30% increase? How do we find the multiplier?

Let's review what a multiplier is. Say I have some number "x."

A "multiplier" is a number I multiply by x in order to take a certain percentage of x or increase or decrease x by a certain percentage.

100% of x would be just 1 * x = x

70% of x would be 70/100 * x = 0.7x

225% of x would be 225/100 * x = 2.25x

3% of x would be 3/100 * x = 0.03x

0.17% of x would be 0.17/100 * x = 0.0017x

Now, that's just taking a percentage "of" x.

If we want to *increase* x by a percentage or express a percentage more than x, we just add the percentage increase to 1.

Examples:

70% increase in x = 100% of x + 70% of x = 1x+ 0.7x = (1 + 0.7)x = 1.7x

So 1.7x represents a 70% increase in x.

43% increase in x would be: x + 0.43x = 1.43x

200% increase in x would be: x + 2x = 3x

If we want to decrease x by a percentage or express a percentage less than x, we just subtract that percentage from 1:

70% decrease in x = 100% of x - 70% of x = 1x - 0.7x = (1 - 0.7)x = 0.3x So 0.3x represents a 70% decrease in x.

Notice that this multiplier .3 is the same as (30% of x).

43% decrease in x would be: x - 0.43x = 0.57x a 98% decrease in x would be: x - 0.98x = 0.02x

So, the multiplier for a 30% increase in x is:

x + 30/100 * x = x + 0.3x = (1 + 0.3)x = 1.3x

This makes sense, because 1.3x is greater than x, and when we increase x by 30% we should have more than x.

0.3x would be 30% OF x

30/100 * x = 0.3x

So if we had 100, 30% of 100 would be 0.3 * 100 = 30.

But increasing 100 by 30% would be 100 + (0.3 * 100) = 1.3 * 100.

Q: How do we know that .78 represents a 22% decrease and .84 represents a 16% decrease? How do we know whether we have an increase or a decrease?

When we have sequential percent changes, we can express each percent increase, decrease, or "of" of a number as a multiplier. The sequential product of all the multipliers together will either be a number less than one  or a number greater than one.

If the product is a decimal less than one, we have a decrease.

And percent decrease is:

(1 - product)

So when we get .78, we know that's a decrease, and the amount of decrease is: (1 - .78) = .22, which is 22%. So we have a 22% decrease.

When we get .84, we know that's a decrease, and the amount of decrease is:

(1 - .84) = .16, which is 16%. So we have a 16% decrease.

If the product is decimal greater than one, we have an increase

And the percent increase is:

(product - 1)

So say we had a a product of 1.68. That's an increase, and the amount of increase is:

(1.68 - 1) = .68, which is 68%, so we have a 68% increase.

If our product is one exactly, then that's just 100% of our original, so we had no change.

More examples:

Resulting product of .77: decrease of (1 - .77) = .23 = 23%
Resulting product of .01: decrease of (1 - .01) = .99 = 99%

Resulting product of 1.9: increase of (1.9 - 1) = .9 = 90%
Resulting product of 8.4: increase of (8.4 - 1) = 7.4 = 740%
Resulting product of 2: increase of (2 - 1) = 1 = 100% <---(that's an increase of 100%...i.e., an exact doubling of our original)

Example: What is the multiplier for: 60% of X increased by 60% then decreased by 60%?

.6X = 60% of X

...increased by 60% = 1.6(.6X)

...decreased by 60% = .4(1.6(.6X))

= .384X

This final number represents 38.4% of our original X. Or we could say that it is a (1 - .384) × 100%  = 61.6% decrease in X.