Volume Unit Cost Before
44 14.11 n/a
9 10.46 8.58
37 9.61 7.88
20 8.26 6.77
78 7.36 6.04
0 3.65 n/a


Can someone show me how to multiple column a*b if column c is "n/a". And column a*c if it has a number, with suming them at the end?

Thanks