LCD state for Least Common Denominator
The given fraction are :

For the least common denominator, first find the LCM of all the denominator of the given values:
Denominator are : ( 20, 18, 90)
LCM of (20,18, 90) = 180
So, the fraction will value can be written as :

Thus, the fraction will convert as :

The least common denominator is 180
Answer : LCD of 11/20, 1/18, 13/90 is 180