A jar contains only red and blue marbles in a ratio of 7:3. If there are 9 blue marbles, how many red marbles are there?

Correct answer

D. 21

Explanation

The ratio 7:3 means for every 3 blue marbles there are 7 red. 9 blue ÷ 3 = 3 groups, so red marbles = 7 × 3 = 21.

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