Question

What is the LCD of 2/7 and 9/10

Answer 1

Assuming LCD means Lowest Common Denominator
Assuming by "and" you mean adding the two

Well, you go by the multiples of 7;
7, 14, 21, 28, 35, 42, 49, 56, 63, 70
Multiples of 10;
10, 20, 30, 40, 50, 60, 70, 80, 90, 100

Straight away you can see the lowest common multiple is 70,
Now to go about adding them,
2/7 * 10 (both sides) , 20/70
9/10 * 7 (both sides) , 63/70

Then you add the two; 20/70 + 63/70  - now we can add them because they share the same denominator
20/70 + 63/70 = 83/70 = 1 whole 13/70 (simplified)

The LCD is 70

Quick tip, to find a common multiple (not necessarily the lowest) multiply the two denominators. 

Answer 2

The least common denominator or LCD is 70 because for
10=10,20,30,40,50,60,70,80
7=7,14,21,28,35,42,49,56,63,70,77
and 70 is the least common denominator of 10 and 7 so 70 ims your answer