Rate of Interest? Ugh Help!

[Tashwitz]

Bob plans to invest $7580. Fine the annual rate of interest required for the fund to grow to $7769.50 in 4 months.

I honestly don't know how to work this problem and wondered if someone could walk me through it. I have looked through my notes and just can't figure this one out =(

 

[Denis]

Bob plans to invest $7580. Fine the annual rate of interest required for the fund to grow to $7769.50 in 4 months.

7769.50 - 7580 = 189.50 = interest for 4 months:
so interest for 1 year = 189.50 * 3 = 568.50;
568.50 / 7580 = .075 : or 7.5%