conditional ad after 5th, 15th post

[Dave-ahfb]

I have it working fine after 5 posts with
[code]<vb:if condition="$post['postcount'] % $vboptions['maxposts'] == 5">;/code]

and it puts the 2nd ad under 15 if I change the 5 to 15.

It also places ads under 30 and so on (for those with extended posts per page)

Is there a way to combine the conditional statement to include the ad spots I want like 5, 15, 25, 35 rather than right a new conditional which seems to have less control?