How to ignore cell formula when the selected cell is blank

Hi! What’s the formula In numbers to ignore a formula if the selected cell is blank?

for example:

column “C” has 60 blank cells that I will only fill once I get a deposit from a client.

column “D” has 60 formulas of (C#)0.029x0.30. This formula fills in all spaces as “0.30” even when Column “C” is blank.

so technically C2 will be with D2 and C3 will be with D3 and so on.

I am only wanting the formula in column “D” to calculate when column C has a number in it.

thanks!