Suppose I would like to do something like this.

Instead of calculating interest by hand, of course we should use formula to do that.

We can put =A2*B2/100, =A2*B3/100 and =A2*B4/100 to C2, C3 and C4 respectively. But it is troublesome to prepare different formulas for each row.

Instead, we can use $ to lock both column index and row index for A2, so when we "paste" the formula, Excel will update the row index of B for us.

We put =$A$2*B2/100 to C2 and copy C2.

When we paste the copied C2 to C3, we will get =$A$2*B3/100 without additional modification.

But still, we need to keep pasting the formula to other cells. We can simply drag the small dot at the right bottom corner of the cell to the cells you want it to copy to.