Answer:
36 years old
Explanation:
Prof. Hall's age meets three criteria:
1. Her age is a multiple of both 4 and 10. This means her age can be 40, 80, 120, etc. Any number that you can get by multiplying 4 and 10 together.
2. Her age is one away from a multiple of 9. This means her age could be one less or one more than a number that you can get by multiplying 9 with another number. For example, one less than 9 times 4 is 35, and one more is 37.
3. She is younger than 90 years old. This tells us that her age can't be 90 or more.
Taking all these conditions into account, the age that satisfies all of them is 36. It is a multiple of 4 ;(4 * 9 = 36), it's one away from a multiple of 9; (9 * 4 - 1 = 35), and it's less than 90. So, Prof. Hall's age is 36 years old.
Hope this helps!