How to calculate the average wage increase?
This is a very standard % change over time formula.

It is simply:
Change Between Years / Original Value

$ΔW_t$ means the change in wage from year $t-1$ to year $t$ (where $t$ represents an arbitrary amount of time). You can easily calculate that by: $ΔW_t=W_t−W_{t−1}$.

Thus it is simply saying % change over time is $\frac{New - Old}{Old}$.

Middle school math with more symbols.