Question 201179
To save for college tuition, the parents of a preschooler invest $5000 in a bond that earns 6% annual interest compounded monthly. In approximately how many years will the investment be worth $15,000? 
15000 = 5000*(1 + 0.06/12)^n
1.005^n = 3
n*log(1.005) = log(3)
n = log(3)/log(1.005) = ~ 220.3 months
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The amount does not matter.  Any amount will be 3x in that many months at 6%.