A teacher had 21 red pens. if the ratio of red pens to blue pens she owned was 3:1, how many pens did she have in total

There are 21 red pens. For every 3 red pens, there is 1 blue pen.

Let's make these into a fraction equation.

21/p=3/1

p is the number of blue pens we have yet to find.

First, cross multiply.

21x1=21

px3=3p

Now, we have an equation of

21:3p

We set it equal to 1, and we solve it.

21:3p = 1

(21:3p) * 3p=1*3p

21=3p

21:3 = (3p) : 3

7=p

So, if there are 21 red pens, there are 7 blue pens.

We add these together to find the total number of pens the teacher has.

21+7=28

The teacher has 28 pens total.

She had 21:7 because if you multiply 3:1 by 7 each it is 21:7 so she had 7 blue pens