So one supplier comes on the 5th day, 10th day, 15th day, and so on...
another on the 6th, 12th, 18th, and so on...
last supplier comes on the 7th, 14th, and so on...
So we want to know when they will all visit on the same day.
Hmmm so eventually they'll overlap :) but when?
One thing you can do is simply multiply 5 * 6 * 7, that will tell you that in 210 days they will all visit on the same day.
But they want to know the NEXT time they'll all visit again, so there might be a lower number hmmmmmmm :O