I'm pretty sure it was III that made you waste time.

My advice is to notice that there are basically three sections to the graph:
a. 1985-1987 with income around 25 000
b. 1988-1991 with income around 35 000 - 40 000
c. 1991-1995 with income around 65 000

As you may imagine, the average must be somewhere close to section b. The reason why I'd go for average greater than 40k is that you have three years in section a and four years in section c. This will tilt the balance toward section c, with higher earnings. A secondary reason is that the difference between section a and section b is around 15k, while the difference between section b and section c is over 25 000. This also moves the average to "higher grounds".