Math calculation

Hi all,

I need to make some calculations and I can't get it working.
I have 4 textboxes where  I store :
a = cycle time (type real)
b = Quantity (integer)
c = time (integer) I need to devide with 60 to get the number of minutes
d = cavities (integer)

My formula should look like this:
e = ((a * b) / c ) / d

I managed to to the calculation until here: e=(a*b) / c but when I want to divide the result to d, i'm getting an error (not a floating point).

Can someone help me achieve this or tell me the easiest way.
The value will not be stored in the database, will only be calculated on the fly when you enter  new record or edit an existing one.

Thank you.

Alin