###Human: Shawn collected 40 plain pebbles. He painted 9 pebbles red and 13 pebbles blue. He then divided the remaining pebbles equally into 3 groups, and painted them purple, yellow, and green. What is the difference between the number of blue and yellow pebbles?, ### Assistant: After painting the red and blue pebbles, Shawn had a remainder of 40 - 9 - 13 = 18 pebbles.
He painted 18/3 = 6 pebbles yellow.
The difference between the number of blue and yellow pebbles is 13 - 6 = 7. 
 So the answer is :  7