Question

What's the common denominator between 210 and 770

Answer 1

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.