There are 18 balls in a jar

There are 18 balls in a jar. You take out 3 black balls without putting them back inside, and now the probability of pulling out a black ball is 1/3. How many blue balls were there in the beginning?

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #48221