Try this:
in D1
=IF(A10,IF(B10,A1+(A1*B1)+C1,A1+(A1*7%)+C1),0)
copied down

A1 - amount
B1 - if left empty then the tax will be 7%, otherwise tax in B1
C1 - gratuity
D1 - total

Hope this helps!
JG

"Slow Learner" wrote:

Hi there can someone help me...
worksheet has 4 colums, column 1 contains full price of article (known
amount), column 2 has tax amount (not always known), column 3 has gratuity
amount (sometimes known), column 4 has total without tax but with gratuity.

Tax, if unknown, is calculated at 7% of amount without gratuity.

What would the formulas be to have my calculations become automatic on input
of full amount?

Thank you