Simple probability question!?

Question

If the probability of choosing 2 red marbles without replacement from a bag of only red and blue marbles is 3/55 and there are 3 red marbles in the bag, what is the total number of marbles in the bag? Thanks for your help and the answers is 11 but I don't understand. Please explain!

Answer 1

Number of red marbles = 3.

Probability of choosing 2 red marbles from 3 red balls without replacement from a bag = 3C₂ / nC₂.

Therefore total number of marbles = 11.