I am looking for a formula to include in a spreadsheet for the following:

I have two equal length columns of no more than 50 numbers.

Say:

A B
1.5 9.8
1 11.7
..5 14
0 16.7

I have two reference cells with values 1 and .62.

If I look up column A reference "1" the answer I am looking for in column B
is "11.7" which I can work out using the Lookup function.

With the second number .62, the answer I am looking for needs to be 13.724
which is the pro-rata difference between the values in A and B.

Any help would be appreciated.

Thanks,
Mal