Simple interest earned on a sum of money for 4 yrs at 7.5% p.a. exeeds that on the same sum for 3 1/2 yrs at 8% p.a. by $90.
A) find the original sum of money
B) if the original sum of money accumulates to $4612.50 in 5 months at simple insterest, find the interest rate per annum