creating a semi-simple formula in Excel?

I have 3 columns.

Column A (hourly rate)

Column B (weekly rate)

Column C (daily total)

How do I create formula in column C so that it calculates a total differently depending on whether there is an amount in column A vs column B.

for example: 

if column A has an amount in it, then I want column C to calculate a formula to tally what's in column A*8.

if column B has an amount in it, then I want column C to calculate a formula to tally what's in column B/5.