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