The formula for simple interest is:
Total = Principal(1+ Rate x time)
You are given:
Total = $100
Principal = $20
Rate = 4%
You are asked to find the time.
Replace everything known in the formula:
100 = 20(1 +0.04 x Time)
Now solve for Time:
Divide each side by 20:
5 = 1 + 0.04 x Time
Subtract 1 from each side:
4 = 0.04 x time
Divide each side by 0.04:
Time = 4 / 0.04
Time = 100 years.
