Increase or decrease a number by "x" percentage

Question

How do I increase or decrease an "X" number by a certain amount of percentage say, 2% or 3%?

Case scenario: My value is 5 - I want to increase this number by 2% - so the final value will be 5.1 after 2% percentage increase.

And the same would be if I want to decrease this number by an "X" percentage.

I hope that makes sense, thank you.

Answer 1

A percent is just a fraction of 100, so multiply by 1 + your percent over 100:

>>> x = 5
>>> x *= (1 + 2/100)
>>> x
5.1

Another way to think of it is incrementing x by x times that percent (these operations are algebraically equivalent):

>>> x = 5
>>> x += (x * 2/100)
>>> x
5.1

Answer 2

If you want to increase by a percent, multiply the number by (1+{percent}). To decrease by a percent, multiply the number by (1-{percent}). Given your example:

• Increase 5 by 2%: 5*(1+0.02)=5*1.02=5.1
• Decrease 5 by 2%: 5*(1-0.02)=5*0.98=4.9.