There is a jar of green and red marbles. If there are 21 marbles total and the ratio of green to red is 3:4, how many red marbles are there?

12 red marbles

Step-by-step explanation:

You make an equation: 3x+4x=21

Then, you get 7x=21.

By using algebra, you get x=3.

Then you muliply 3 by 4 since the ratio of red marbles is 4.

So you get 12.

There are 12 red marbles

Step-by-step explanation:

green : red : total

3 4 3+4=7

We want a total of 21

21 / 7 = 3

Multiply by 3

green : red : total

3*3 4*3 7*3

9 12 21