if function

[hitesh]

dear

bill july-07, coll july-07 ,w/o jul-07 ,bill aug-07,coll aug-07,w/o aug-07

210 140 210
366 370 366 300
210 210 210 140

if bill for july is 210 and i collect the amt 140 than the w/o july-07 must
be 70 (this 70 only for one cell). if bill for july is 210 and i collect the
amt 500 than the w/o july must be 100,w/off aug 100 upto 5 month it should be
100.

i know that ther is some greater and less than formula using if, but i dont
know how to use.
so pls help.

regards
anil