You want to be a millionaire when you retire in 40 years. How much do you have to save each month if you can earn a 12% annual return? How much will you have to save if you wait 10 years?

Answer:
Today: $85
10 years: $286.13

I have a test coming up and just need to know how to solve them, I can't seem to get it right.

Thanks

The problem fits the pattern of an ordinary annuity. Google "future value of an ordinary annuity" for more insight, but the formula you need sets up as

A \ \times \ \left( \frac{(1+r)^n-1}{r} \right)

A is the amount being invested each month, n is the total number of months, and r is the monthly interest rate. Note that you are given an annual rate, so you'll need to divide it by 12.

Use the formula to verify that $84.99 and $286.12 fall just short of the target, whereas $85 and $286.12 have the legs to cross the finish line.