Question

You have piano lessons every fifth day and cooking lessons ever sixth day. Today you have both lessons. In how many days will you have both lessons on the same day again

Answer 1

In thirty days. The least common multiple of the two is 30 (5x6), and this would be the next day that you would have both.

Answer 2

Make a list
5th day so 
5,10,15,20,25,30,35,40

6th day is 
6,12,18,24,30
the correponding day is 30 so the answer is 30