Assign specific value to random

Hello i try to use arc4random function to estract sono text from a file, but the file have 100 lines and i'd like that the random function estract only a specific random line such as 6 - 12 - 18 - 24 - 30 . . . multiple of 6.
it's possible?

This is my actual code

Code:

int numLines = [rowFile count];
	
	
	
	int row = arc4random() % numLines;