
Estimation
I have no idea on how to even start this question!Please help
A researcher has funds to buy enough computing power to numbercrunch a problem in 5 years. Computing power per dollar doubles every 21 months.
(a)When should he buy his computers to have finished the problem as soon as possible? Give your answer in months, accurate within 0.1 months

Let T be the time it takes the computer to solve the problem. Let t be the time from now until when the researcher buys his computers. We wish to minimize t + T. Now t is just going to increase at the normal rate. We'd like to write T in terms of t. Can you think of a formula for T in terms of t? What conditions should it satisfy?