How do I increase an amount by a percentage on a calculator

For example: 40 increased by 7% or 40 increased by 20%

Convert the %number (7% = .07, or 20% = .2). Then add 1.0. You get 1.07 or 1.2. Multiply that number by the base number (40) in your example.

1.07 x 40 = 40.28
1.20 x 40 = 48

convert the percentage to a decimal fraction, add 1, and then multiply that by the value you want to increase.

7% is 0.07, so to increase anything by 7%, multiply by 1.07. For 20%, multiply by 1.20, and so on.

For your examples: 1.07 * 40 = 42.8 and 1.20*40 = 48

The same idea works for decrease if you subtract the decimal fraction from 1 instead of adding. For example, 1 - 0.20 = 0.8 is the value to multiply by for a 20 % decrease. But, that's another lesson...

Multiply by 1 + %/100
40 increased by 7% =40*(1+7/100) = 40*1.07
40 increased by 20% = 40*(1+20/100) = 40*1.20