Can you help with my math homework?

Question

A bag contains only red and blue marbles. There a total of 36 marbles in the bag. There are 5 red marbles for every 4 blue marbles. A student removes 1 blue marble from the bag. The student reasons that there are now 5 red marbles for every 3 blue marbles since 4-1=3. What is the students error?

Answer 1

The student would be correct if there were 9 marbles originally (5 red, 4 blue) but there are 36 (20 red, 16 blue). The new ratio is 36/15 or 12/5