Alright, so you have the basic formula- good. You have the A value (400), the interest rate r (7.5% -> .075 in decimal), and the final P value (8500). So, we only need to solve for t.
8500 = (400)(1+.075)^t /400 /400 21.25 = 1.075^t logarithms are the inverse of exponents, basically, if you have an example like y = b^x, then a logarithm inverts it, logy(baseb)=x Makes sense if you consider a power of ten. 1000 = 10^3 if you put logbase10(1000), you'll get 3. Anyways, though, to solve the problem make a log with a base of 1.075 in your calculator log21.25(base 1.075) = t also, because of rules of change of base (might want to look this up to clarify), you can write this as log(21.25)/log(1.075) = t Thus, t is 42.26118551. Rounded to hundredths, t=42.26
