Question

Stephen has art lesson every 6 days.And a swim lesson every 9 days.He has both art and swim lesson on Tuesday.On which day of the week will he have both lessons again?

Answer 1

6 and 9 go into 18
tue - tue = 21 days
- 3 days = sat

Answer 2

The least common multiple would give you the answer you are looking for.
Multiples of
6: 6, 12, 18, 24, 30, .....
9: 9, 18, 27, 36, .....
He will have a lesson on the same day in 18 days.
Starting on Tuesday we need to go 18 days. 14 days would be another Tuesday, Wednesday = 15, Thursday = 16, Friday = 17, Saturday = 18