If cell is not blank calculate, else calculate another cell

Hello,

I have a table where I want a formula in cell C2 to divide the amount in A2 (e.g. by 4) if populated, or if blank to divide the amount in cell B2 instead.
[TABLE="border: 0, cellpadding: 0, cellspacing: 0"]

[tr]

[TD="width: 23"] [/TD]
[TD="width: 58"]A[/TD]
[TD="width: 49"]B[/TD]
[TD="width: 75"]C[/TD]

[/tr]

[tr]

[td]

1

[/td]

[td]

CAT 1

[/td]

[td]

CAT2

[/td]

[td]

1Q

[/td]

[/tr]

[tr]

[td]

2

[/td]

[td][/td]

[td]

100

[/td]

[td][/td]

[/tr]

[tr]

[td]

3

[/td]

[td]

125

[/td]

[td][/td]

[td][/td]

[/tr]

[/TABLE]

Anybody have any ideas how I can accomplish this?

Thanks in advance,

zdjbel

Thank you so much Carim, didn't realize it was this simple, I was obviously on the wrong path trying to do it with ISBLANK.