Answer:
- least: 140
- others: 280, 420
Step-by-step explanation:
It can work to start by factoring the numbers:
20 = 2²·5
28 = 2²·7
The LCM of 20 and 28 is 2²·5·7 = 140, the product of unique factors to their highest power. 140 is the least common denominator of the two fractions.
Any multiple of 140 will also be a value that can be used as a common denominator. Shown above are 2·140 = 280, and 3·140 = 420. The product of the given denominators, 20·28 = 560, is also another possible common denominator.
