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.
