Question

A bag of marbles has twice as many blue marbles as green marbles, and the bag has at least 57 marbles in it. At least how many green marbles does it have?

Answer 1

Let's call the number of green marbles "G"

So we have G number of green marbles and 2G blue marbles (that is, two times as many green marbles as blue marbles)

So we can say G + 2G ≥ 57
or
3G ≥ 57

divide both sides by 3
G ≥ 19

You have at least 19 green marbles