What is a multiple of 6,9,18 between 61 and 107

There are two possible answers: 72 and 90.

We can find the answer by looking at the largest number among the three of them: 18. The only multiples of 18 between 61 and 107 are 72 and 90. We can check that both numbers are also multiples of 6 and 9:

90 : 6 = 15
90 : 9 = 10

72 : 6 = 12
72 : 9 = 8