There are 120 bricks in a box . the bricks are red blue or green. there are 1/3 red bricks . 1/4 blue bricks

So you need the fraction of green ones?

= 1 - 1/3 - 1/4
= 1 - 4/12 - 3 /12
= 12/12 - 4/12 - 3/12 
= 5/12

and the numbers of each brich are  1/3 * 120 = 40 red 

1/4*120 = 30 blue and 5/12 * 120 =  50 green

What you have to do is find what 1/3 is of 120 and 1/4 of 120.
120/3=40, so each third is 40 bricks
120/4=30, so each fourth is 30 bricks
Add those together
The total of the red bricks and blue bricks is 70
120-70= 50

There would be: 40 red bricks, 30 blue bricks, and 50 green bricks

The correct answer is c