useing the rule of 72 estemate how long it will take an investment of $400 to double if it is invested at 4.5%/year compounded monthly. (show work)

I * Y = 72

What that means is this:

(interest rate in %) x (number of years to double) = 72

4.5 * Y = 72
Y = 16 years

The formula is approximate, but pretty good.

The actual factor that the invested amount would increase in 16 years with monthly compounding is 2.0517. the $400 would become $821 in that time.