The ratio of yellow marbles to green marbles is

Question

7 yellow marbles to 2 green marbles.
If there are 21 yellow marbles,
how many green marbles are there?

Answer 1

7 y/2 g = 21 y/x g
7/2 = 21/x
7x = 42
x = 6

Or

just notice that 21 yellows is 3 times the 7 yellows, so
wouldn't you need 3 times the 2 greens? , which would be 6