What's the common denominator between 210 and 770

Suppose you want to combine the fractions 13/210 and 29/770. What's the LCD?

Factor both 210 and 770: 3*7*10 and 7*11*10. Form the LCD using the common factors 7 and 10 and the unique factors 3 and 11. The LCD is thus 3 (7) (10) (11), or 2310.

Then 13/210 and 29/770 become 143/2310 and 87/2310. We can add these because their denominators are the same.

So the desired LCD is 2310.