To solve these problems you need two formulas;

1. I = Prt; I = interest, P = principle, r = rate of interest, t = time in years

This is the formula for computing **simple interest**

2. I = A - P; I = interest, A = total amount, P = principle

This is really more intuitive.

Combining the two yields A-P = Prt

5) From the information given and plugging these values into the above formula;

A - 8400 = 8400(10)r; A = 10(12)122.50 = 14700

(14700 - 8400) / 84000 = r = 0.075 = 7.5%

7) Since A = 2P and I = A - P, then I = P

P = Prt; rt = 1; r = 12.5% = 0.125

t = 1 / 0.125 = 8 years

8) Now A = 3P which means that I = 2P

2P = Prt and t = 25;

2 = 25r

r = 0.08 = 8%