That’s not how compound increases are measured.

We can use the compound interest formula for this.

A = P * (1 + r) ^ t

• A is the final amount.
• P is the starting amount (the principal).
• r is the rate (as a proportion, so 50% would be 0.5).
• t is the time.

To figure out the annual increase for the whole time we can plug in what we know and solve for what we don’t:

7.25 = 0.25 * (1+r) ^ 80
29 = (1+r) ^ 80 years
1.043 = 1+r
0.043 = r

So that’s about 4.3% increase per year over the 80 years.

Now we can see what we would have as minimum wage if it had continued over the past 15 years:

A = 7.25 * (1+0.043) ^ 15
A = 7.25 * 1.043 ^ 15
A = 7.25 * 1.88
A = 13.63

So that’s a $13.63 minimum wage.