The recipe for cereal squares uses the ratio 20 large marshmallows to 3 cups of cereal. The ratio of mini-marshmallows to large marshmallows is 3 to 1. How many mini-marshmallows will Jamie need to mix with 6 cups of cereal to make cereal squares?

The relationships are:

20 large marshmallows = 3 cups cereal

3 mini marshmallows = 1 large marshmallow

Therefore if there are 6 cups cereal, so we also get 40 large marshmallows. Since for every 1 large, we need 3 mini, hence:

mini marshmallows = 40 * 3 = 120