If the original number is 15 and the new number is 5, what is the percent decrease?

When a quantity grows (gets bigger), then we can compute its PERCENT INCREASE:

[beautiful math coming... please be patient] PERCENT INCREASE=(new amount−original amount)original amountPERCENT INCREASE=(new amount−original amount)original amount

Some people write this formula with 100%100% at the end,
to emphasize that since it is percent increase, it should be reported as a percent.

So, here's an alternate way to give the formula:

PERCENT INCREASE=(new amount−original amount)original amount⋅100%PERCENT INCREASE=(new amount−original amount)original amount⋅100%

Recall that 100%=100⋅1100=1100%=100⋅1100=1 .
So, 100%100% is just the number 11 !
Multiplying by 11 doesn't change anything except the name of the number!
Hope this helps