Suppose, i have 3 incentive slabs

No.of sale            incentive slab

1-10                     no.of sale*8
11-20                   no.of sale*10
21 and above        no. of sale*15

What formula i have to insert in the incentive column , so i will get the
incentive of every salesman if i fill only the no. of sale in the column left
to incentive slab column ?If this works i will be able to save a lot of time
and energy so please help and explain in details with examples.