20
A common denominator of 20 and 30 is 600. This is NOT the least common denominator, though.
20
20
20
9/10 and 2/20
First of all you need to line them up. 8/20 - 2/5 Next you need to find a common denominator. 20 is a common denominator between the 2. Then you change both denominators to the common one. 8/20=8/20, 2/5 x 4/4= 8/20 {Since 5x4=20, you have to do that on both sides to get the common denominator.} Then you subtract 8/20 - 8/20 = 0. So the answer is 0
A common denominator, though not the least one, is 12*20 = 240.
A common denominator, though the least, is 260.
20
The answer is 20
1/2 3/2
The common denominator of 5/20 and 2/3 is 60. Of course, 5/20 is really 1/4 so it will eventually reduce down to 12.