If a boy has music lessons on every third day soccer sixth and karate ninth which will he have all three?

To find the day when the boy will have all three, we need to find the least common multiple of 3, 6, and 9.

The multiples of 3 are: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, ...

The multiples of 6 are: 6, 12, 18, 24, 30, ...

The multiples of 9 are: 9, 18, 27, 36, ...

The least common multiple of 3, 6, and 9 is 18. This means that the boy will have all three lessons on every 18th day.