Explanation:
the area of a rectangle is
length × width.
our job here is to find all integer values for length and width, so that their product is 90.
that means that length and width are both factors of 90.
the factors of 90 are
1 × 90
2 × 45
3 × 30
5 × 18
6 × 15
9 × 10
10 × 9
15 × 6
18 × 5
30 × 3
45 × 2
90 × 1
the prime number grouping
is
2 45 (can't be divided by 2 anymore, so,
next prime factor)
3 15
3 5 (can't be divided by 3 anymore, so
next prime factor)
5 1 (the end)
so, besides 1 and these prime factors, the only other factors are combinations of these prime factors :
2×3 = 6
2×5 = 10
3×3 = 9
3×5 = 15
2×3×3 = 18
2×5×3 = 30
3×5×3 = 45