If you are looking for a more general approach, then use column A for
Number of bikes, column B for Duration, column C for Unit rate (per
bike per week) and column D for the Total. Put appropriate headings in
A1 to D1, then in A2 you can enter 15, in B2 you can enter 52, in C2
you can enter 250 (and format the cell as currency), then in D2 you
can enter this formula:
=A2 * B2 * C2
to get your answer.
If you just want to use Excel as a calculator, then in A1 you can
enter:
=15 * 52 * 250
Hope this helps.
Pete