The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90. Calcating the factors of 90 is straightforward. First, every number is divisible by itself and 1.
Try 1. 1 x 90 = 90, so put these into our factor list.
1
...
90
Take 2. 2 x 45 = 90, which means we add these to our factor list.
1
2
...
45
90
Now take 3. 3 x 30 = 90, so now we put these in our list of factors.
1
2
3
...
30
45
90
Next try 4... no luck. 90/4 isn't a whole number. So we skip 4.
Now try 5. 5 x 18 = 90, so put these into our factor list.
1
2
3
5
...
18
30
45
90
Try 6. 6 x 15 = 90, which means we add these to our factor list.
1
2
3
5
6
...
15
18
30
45
90
Take 7... no. 90/7 isn't a whole number. So we skip 7.
Now take 8... no luck. 90/8 isn't a whole number. So we skip 8.
Next try 9. 9 x 10 = 90, so put these into our factor list.
1
2
3
5
6
9
...
10
15
18
30
45
90
Now, since we don't have any more numbers to try, we're done!