Greetings!
"What is the LCM of two numbers if one number is a multiple of the other"...
The LCM (The Lowest Common Denominator) would be the number itself, multiplied by one.
For Example:
3/4+3/20
Multiples of the Denominators:
4:4,8,12,16,20,24
20:20,40
4*5=20
20*1=20
The LCM would be 20.
Hope this helps.
-Benjamin