A bag contains only red and blue marbles. The probability of drawing a red marble is 3/5. If there are 18 red marbles in the bag, how many blue marbles are there?

✓ Correct Answer: Option B

Probability of red (3/5) equals red marbles (18) divided by total marbles. So, 18 / Total = 3/5. This means Total = 18 * 5 / 3 = 30 marbles. The number of blue marbles is Total - Red = 30 - 18 = 12.