If those are all denominators, the LCD is 360.
A common denominator is 72.
120
72
120
The Greatest Common Denominator is 6
21/24 and 20/24
The Least Common Multiple (LCM) for 18 24 20 is 360.
21/24 and 20/24
The common denominator is 72, so 5/18 = 20/72, and 7/24 = 21/72. Now you can subtract the numerators [20-21 = -1]. So the answer is -1/72.
For 18 and 56: 504 For 1/8 and 5/6 : 24
The LCD of 1/3 and 1/8 is 24.
11 over 24 (11/24) Here's how: You need to find a common denominator. The lowest common denominator is 24, but you can use any common denominator to subtract. So convert 5/6 to 24ths: multiply numerator and denominator by 4 = 20/24, then multiply numerator and denominator of 3/8 by 3 = 9/24. Now subtract 9 from 20 = 11, which is the numerator of the difference: 11/24. This is already in simplest form.