What is the least common denominator of 2/7 and 5/4

28 is the LCD. You can always get a common denominator by dividing the numbers 7 x 4 = 28.

To find the LCD I look at the multiples of the larger number (since there will be fewer of them) As I do this I check to see if the other number is a factor of it

7 (4 isn't a factor), 14 (4 isn't a factor), 21 (4 isn't a factor), 28 (4 IS a factor) !