Excel 2007 : Formula to Differentiate Empty Cell from Cell returning Blank as result

Hi, the question may be slightly confusing....

-------- A ----------B
1 ------ 3 --------- 15
2 ------ 0 --------- 0
3 ------ 7 --------- 35
4 ------ 2 --------- 10
5
6

For example, the formula from B1:B4 = (INDEX(A:A,ROW())) *5 i.e B1 = A1 *5
As you can see, Row 2 returns a "0" via formula, while Row 5 & 6 are blanks

I wanted a formula to display the row number of the last row that contains formula in Column B, which should be "4" in the example.

Is there a way to do that? Thanks in advance!