MATH HELP
posted by Sammy .
Derrick is investing $1,000 at 5% interest and Anna is investing $750 at 7% interest. Both interest rates are compounded annually. When will they have the same amount saved? (Hint: 1.05t/1.07t =(1.05/1.07)t )

we want
1000*1.05^t = 750*1.07^t
(1.05/1.07)^t = 3/4
.9813^t = .75
t = log(.75)/log(.9813) = 15.24 years
Extra credit: after that point, who gets the larger amount? 
calculate the compound interest on an investment of $18,000 at 8%, interest compounded quarterly, for 15 months.