calculations in tables

I am doing calculations in tables in Word but am having trouble with part of
the calculation. In cell A4, I want to insert multiple IF tests. I want to
say that if cell A3=10, make the number in cell A4=1, if cell A3=12, make the
number in cell A4=3, if cell A3=14, make the number in cell A4=5, etc., etc.
Cell A3 could calculate to any of five numbers and basically if it calculates
to a double number, I want it to add the two numbers together and put a
single number in cell A4. If, however, cell A3 is only a single number, I
just want that single number copied to cell A4.

Can someone please tell me how to do this. I have limited knowledge of
calculations in Word and am just learning as I go along. So far I have
managed but this one has got me beaten.

Thanks in anticipation of any replies.