Perimeters and Measurements...Urgent assistance!!!

Here are a list of problems in which I seek assistance on...if you could include the calculation (ways/steps of getting the answer, that would be highly appreciated)

1. Estella borrowed $900. Interest on this loan is $108. How much will estella have to repay?

2. You are cooking a 5 1/4 pound roast to serve 14 people. How much meat will each person receive?

3. How many 4' * 8' sheets of drywall do you need for the walls of a 12' * 20' room, 8ft high?

4. If ceramic tiles are 8" * 4", how many tiles would it take to cover an area 4' * 6'?

5. If a customer buys 20 2*4's 8ft long, at $4.75 each, how much would the 2*4's cost?

thanks :-)