Value from 1 to 80, divided by a number from 1 to 80

This has brought me to a hault. The c128 has 80 columns (VDC Mode). how could i like. divided 80 by 15? 40 by 7.. neither values will inc higher than 80 or lower than 1, cud be 80 by 2??
Any kind of down & dirty method, please let me know. I appreciate you taking the time to read this, and help this program get closer to complete...