The answer is 90. It can be divided by 2,3,5,6, and 9. The other numbers in the choices cannot be divided by all of these numbers to find a whole. 78 can't be divided by 5 to get a whole number, the other numbers can't be divided by 2 to get a whole number. So 90 is your answer.
90/2=45
90/3=30
90/5=18
90/6=15
90/9=10
