I P
Fill Right
 N

We can also fill right. We must select the original cell (and the cells to the right) and select from the Edit menu -- Fill and Right.

A B C
1 =A2+$B$3 =B2+$B$3 =C2+$B$3
2 6 2 5
3 7 10 4
4 9 8 7

If we were to fill right from A1 to C1 we would get the formulas displayed to the left. Notice that the second part of the equation is FIXED or (ABSOLUTE REFERENCE "$ sign" so always references B3 which is 10).

Answers would be A1=16, B1=12, C1=15.