Abag contains only red and blue marbles. there are a total of 36 marbles on the bag. there are 5 red marbles for every 4 blue marbles in the bag. a student removes 1 blue marble from the bag. the student reasons that there are now 5 red marbles in the bag for every 3 marbles since 4-1=3

This reasoning is incorrect. The reason is because there are 36 total marbles, not 9 total marbles. The ratio of 5 red to 4 blue marbles only gives us a total of 9 marbles, which is not correctly representing the real amount of marbles.

If one blue marble is taken away, we are left with a ratio of 20 red marbles to 15 blue marbles. This would be a simple ratio of 4 red to 3 blue.