please help me solving these questions

9) Compute the amount of interest charged when you borrow $420 from your friend who asks you to agree to pay back the loan in 4 months at 20% annual interest rate. 10) You have negotiated a purchase price of $25,200 for a new pickup truck. Now you must choose between two financing options: a) Take a $3000 discount off the purchase price from the dealer and take the car loan to your bank at 5.5% interest, compounded monthly, for 48 months. b) Finance through the dealer directly at 2.5% interest, compounded monthly, for 48 months Which option should you choose? 11) If a three-year term deposit at the bank pays an interest rate of 9%, how much will have to be deposited to earn $800 of interest