The ratio of blue marbles to red marbles in a box is 3 to 2.if there are 12 blue marbles in the box then how many red marbles are there

You need to make a proportion.  Then cross multiply and divide to find your answer.

blue  3     12
red    2      x

(2)(12) = 3x
24  = 3x
x = 8

There are 8 red marbles

I think of this like a fraction where as the number of red marbles is X.

the numerator is blue marbles and the denominator is red marbles

3/2 = 12/x

How many times does 3 go into 12?
Answer 4 times.

4 x 2 = X

Final Answer 8

There are 8 red marbles.

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