Answer:
45 lessons
Explanation:
Here, we want to know the number of lessons that both of them would have completed when they earn pizza and cookies at the same time
From the question, we are told that they earn cookies and pizza when they complete 9 and 15 lessons respectively.
So to know the number of lessons completed, we need to find the lowest common multiple of the number of cookies and the numbers of pizza
That will be lowest common multiple of 9 and 15
This is 45