conditional calculation query - help!

this might be an easy one for some of you:

i need a calculation in cell 4 that will multiply cell 1 or cell 2 with the amount inputed into cell 3.

at the moment i have something that works, however, what i have done is; made a hidden cell (cell 0) with the amount '8' as its default value (for 8 hours per day) and multiplied that by cell 1 - so that if i enter '2' into cell 1 it enters '16' into cell 2.

then cell 2 is multiplied by the amount in cell 3 to show the value in the calculation for cell 4.

i hope you are still with me.

surprisingly, that method works - BUT - i want to be able to free text into cell 2 as well, which i can, however the amount visible says '0'.

how can i make it show either what is entered or the calc from above?

ultimately this method is flawed and i would prefer a simple calc to use!

help.