1st year...
500 = 100%
5 = 1%
2.5 = 1/2% or .5%
So, you have to do 5 x 7 to get 35 which is 7% then you have to tack on the 2.5 to get 7.5%
2nd year...
537.5 = 100%
5.375 = 1%
2.6875 = 1/2% or .5%
So now 5.375 x 7 = 37. 625 + 2.6875 (I will round at the end) = 40.3125
So 37.5 + 40.31 (rounding!) = 77.81 in interest so far.
3rd year...
537.5 + 40.31 = 577.81 = 100%
5.77 = 1%
2.89 (was 2.885 but round up) = 1/2% or .5%
so 5.77 x 7 = 40.39 + 2.89 = 43. 28 = 7.5%
577.81 + 43.28 = 621.09
4th year....
621.09 = 100%
6.21 = 1%
3.11 (was 3.105, but round up)
6.21 x 7 = 43.47 + 3.11 = 46.58
621.09 + 46.58 = 667.67
5th year...
667.67 = 100%
6.68 (round up) = 1%
3.34 = 1/2% or .5%
6.68 x 7 = 46.76 + 3.34 = 50.1
667.67 + 50.1 = 717.77
So as a fact, I will go over next year, so your answer is 6 whole years.
Hope this helped! Hope I'm not wrong!