Question 642054
No starting value is required. You are simply asking how many time periods are required to double a value. Here, 6.25% is .5208% per month. 
Before solving, note there is a "rule of 72." A close guess is that 72/6.25 is the years it takes to double. 11.52 years will be close to the answer. 

2=(1.005208^N)  

ln2=ln(1.005^N)  To solve for a power, we take the natural log of both sides of the equation. 

ln2=Nln1.005

N=ln2/ln1.005208

N=.6931/.0051948

N=133.4218

Recall, N is in months, so we divide 133.4218/12 to get ANSWER 11.1188 years. 

This is 3.3% lower than our guess. The rule of 72 can help you on an exam. For example, if you stopped at N=133 and wrote 133 years, you would know something is wrong.