How do I calculate a percentage increase?

Use the formula: (New Number - Old Number) / Old Number. Then multiply by 100. If your rent goes from $1,000 to $1,100, the difference is $100. $100 / $1,000 = 0.10, or 10%.

To find 20% off, first find 10% by moving the decimal one spot to the left (e.g., 10% of $50.00 is $5.00). Then double it. So 20% of $50 is $10.

How do I reverse a percentage?

If a shirt is $40 *after* a 20% discount, you divide by (1 - 0.20), or 0.80. $40 / 0.80 = $50 original price.

Percentage Math for Real Life

"When will I ever use this?" EVERY SINGLE DAY. From calculating a tip to negotiating a raise, percentages rule the world.

Solve any % Problem

"What is X% of Y?" or "X is what % of Y?". Solve it instantly.

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The 10% Trick (Mental Math)

To find 10% of ANY number, just move the decimal point one spot to the left.

10% of $65.00 = $6.50
10% of $120.00 = $12.00
10% of $4.50 = $0.45

Calculating a 20% Tip

Most waiters appreciate a 20% tip. Just calculate 10% (using the trick above) and multiply by 2.
Bill: $42.00
10% = $4.20
Tip = $8.40

Sales Discounts (30% Off)

You want to buy jeans for $80 that are 30% off.
Option 1: Calculate the discount ($80 x 0.30 = $24) and subtract ($80 - $24 = $56).
Option 2 (Faster): If it is 30% off, you are paying 70%. ($80 x 0.70 = $56).

Percentage Change

Used for stock growth, inflation, or weight loss.

((New Value - Old Value) / Old Value) x 100

If you bought a stock at $50 and it is now $40:
($40 - $50) / $50 = -10 / 50 = -0.20
That is a 20% decrease (or loss).