Imagine 3 columns. Cell A1 reads the quantity, Cell B1 reads the Cost and cell C1 reads the Total. Cell A2 has a number 1 in it. Cell B2 has $10.00 in it and Cell C2 should then have $10.00 in it. If I change Cell A2 (the quantity) to 8, then Cell C2 should change to $80.00. Conversely if have Cell A2 be 1, Cell B2 to read $20.00 Cell C2 should be $20.00.

A B C
1 1 $10 10

A B C
1 2 $10 20

A B C
1 8 $15 120

I hope this clears it up.