The interest I lily gained by the simple interest is:
Recall that simple interest . Given principal P, time t, and interest I, you can solve for rate r. Note that the rate here is per month, so you may have to multiply by 12 to switch it to annual for convenience.
Recall that the compounded amount formula is:
Simply find the compounded amount and subtract the principal from it to find the interest. Remember to switch the time to years. Then find out how much more interest she would receive.