Charlie45 wrote:

Unfortunately there is no quick way around these sort of problems. The most efficient way to solve these is to list two columns and progressively count the factors:

1 90

2 45

3 30

5 18

6 15

9 10

Once the numbers on the left column are about to become larger than the ones on the right column you have determined all factors. Division rules are helpful here.

Pretty neat.

This is a more "traditional" way.

get the prime factorization of the number. Then multply every combination of those numbers.

so for 90:

2*3*3*5

so,

2*3 = 6

2*5 = 10

3*3 = 9

3*5 = 15

2*3*3 = 18

2*3*5 = 30

3*3*5 = 45

2*3*3*5 = 90

so, you have 6,9,10,18,15,30,45, and 90.

plus the original 3 prime factors: 2,3 and 5

plus 1 (duh)

answer is 12