Hi lets see what you have here… ok so this is what you do your looking for 
the lowest common multiple and in order to find it you can do it this way. . .

5
   5, 10 ,15, 20,25, …ect

10
   10 , 20, 30 , 40 , 50 …ect

what goes into them both first?

20 does, so that will be your lowest common multiple also known as LCM