Any multiple of 720
90: 80/90 78/90
A common denominator, though not the least, is 80.
A common denominator is 90.
80
A common denominator, though not the least, is 80.
The least common denominator for 9 and 10 is 90.
880
80
1360
180
You need at least two numbers to find something in common. 90 can be a common denominator of other numbers, but it can't have a common denominator all by itself.
The common denominator of 4/8 and 11/15 is 90.