It's worth learning the formula I=PRT/100 for simple interest. Let y be the original sum (P in the formula stands for principal, that is original sum)

So yx7.5x4/100 - yx8x3.5/100=90

For 2nd part rearrange the formula to give R =(100I)/(PT) and put in I (which you can now work out) P (which is your answer to the 1st part) and T=5/12