To determine when Mya will have both lessons again on the same day, you will list the multiples of each number of days because to show every 4 or 6 days, you will count by 4's and 6's.
When you get to the first number that is the same, that will be the next time she will have both lessons again. Â This is called the least common multiple (LCM).
4, 8, 12, 16, 20, ...
6, 12, 18, 24
In 12 days she will have both lessons again.