Felicia put $175 into a CD that pays 4% interest, compounded semiannually. According to the rule of 72, approximately how long will it take for her money to double?

The compound interest is given by:

A=p (1+r/100) ^n

where;

A=amount

p=principle

r=rate

n=time

therefore the time taken for Felicia's money to double will be:

350=175 (1+4/100) ^n

350/175=1.04^n

2=1.04^n

therefore introducing natural longs we get:

ln2=nln1.04

n = (ln2) / (ln1.04)

n=17.7 years

The answer is 17.7 years