21
As 3 and 7 are relatively prime (their hcf = 1), their lcm is 3×7 = 21.
The Least Common Denominator of 3/4 and 7/8 is 8.
There is none. A least common denominator is to be found between or among fractions. 3 and 7 are not fractions.
It is: 42
The least common denominator (LCD) refers to a multiple that is COMMON to two or more numbers. You have only one number in the question!
1
21
105
168
42