Question

What is the formula for calculating constant percent change?

Answer 1

For the period from `t_1` to `t_2` for some state `f(t)` with a constant percentage change, that constant percentage change is given by:
`color(white)("XX")(f(t_2)/f(t_1)-1)*100%`

Explanation:

The actual change from `t_1` to `t_2` is
`color(white)("XX")f(t_2)-f(t_1)`

The change as a fraction of its value at `t_1` is
`(f(t_2)-f(t_1))/f(t_1) color(white)("XX")=color(white)("XX") f(t_2)/f(t_1) -1`

or,
as a percentage:
`color(white)("XX")(f(t_2)/f(t_1)-1)*100%`