Question

if a number decreases from 200 to 100 what is the rate of decrease

Answer 1

The number decreases from 200 to what?
Let's say it decreases to x.

Percent rate of change is: \(\dfrac{x - 200}{200} \times 100\)

Since the change is negative, it's a decrease.

Answer 2

Oh, I see. It decreased to 100.
Replace x by 100 in the formula above and calculate it.