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.