I need help for this question. Can someone pls help me?

[Charmaine]

The simple interests earned on the sum of money for 4 years at 7.5% p.a. exceeds that on the same sum for 3.5 years 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 interest, find the interests rate per annum.

 

[math_celebrity]

Simple interest = i(n)
Using 4 years at 7.5% (0.075), we get:
Simple interest = 4(0.075) = 0.3

What is p.a.?