9. It is a very good idea to divide by the smallest numbers.
First divide by 2 as many times as you can, then by 3, and so on:
110=2*55 (2 is an obvious factor, since the number ends with 0)
2*55=2*5*11 (5 is an obvious factor since the number ends with 5)
10.a
to find a number with factors 2, 3, 5, just multiply these numbers with any other number you want.
number 1: 2*3*5*4=120
number 2: 2*3*5*9=270
The greatest common factor of these 2 numbers is 2*3*5
any combination of these numbers is a common factor to number 1 and number 2,
so these numbers have in common the factors:
2*3=6, 3*5=15, 2*5=10, 2*3*5=30
that is {6, 10, 15, 30}
b.
a number with 8 as a factor, has also 2 and 4 as factors.
so a number with factors 2, 4 and 8 can be found by simply multiplying 8 by any number.
number 1: 8*5=40
number 2: 8*11=88
number 3: 8*500=4,000
these numbers have in common that they all are multiples of 8 (their greatest common divisor is 8)
