A 20-year-old student decided to set aside $100 on

his 21st birthday for investment. Each subsequent
year through his 55th birthday, he plans to increase
the investment on a $100 arithmetic gradient. He will
not set aside additional money after his 55th birth-
day. If the student can achieve a 12% rate of return,
what is the future worth of the investments on his
65th birthday?

This is what I have so far:
100(A/G,12%,30)+(A/G,12%,10)

I do not understand how to set up this problem!!! Help!!!