There are 7 blue pens, 4 green pens and 6 red pens in a box. One pen is taken at random from the box. Write down the probability that this pen is blue

First of all you need to work out the total number of pens. 7+4+6=17. How many of these 17 pens are blue? 7. If you pick one pen out of the 17, 7 of them would be blue. To work out the probability, you need to divide the number of blue pens by the total number of pens. The chance that one pen of the 17 is blue is 7/17.