The ratio of red marbles to blue marbles in Simran's bag is equal to 1 to 1. If there are 20 red marbles, how many blue marbles are there?

If the ratio is 1:1, then there must be 20 red marbles and 20 blue marbles. If it weren't 1:1, then there would be a different amount of blue marbles. But since they are both 1:1, there has to be 20 red and 20 blue marbles.

~Silver