I want to know the formula to use

1. A 120-day simple discount promissory note for $12,000 with a simple discount rate of 9% was signed on July 14. It was discounted on August 30 at 9.5%. Find the proceeds at the time of the sale.
2. 8. A $15,000 T-bill is purchased at a 3.85% discount rate for 16 weeks. Find (a) the purchase price of the T-bill, (b) the maturity value, (c) the interest earned, (d) the effective rate of interest to the nearest hundredth of a percent.