Below is an instruction that describes a task. Write a response that appropriately completes the request in Swahili. Please answer in Swahili.

### Instruction:
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?

### Response:
 Jumla ya mawe yaliyopakwa rangi ni 9 + 13 = <<9+13=22>>22.
Mawe yaliyosalia baada ya kupaka rangi ni 40 - 22 = <<40-22=18>>18 mawe.
Kwa kuwa kulikuwa na vikundi 3, kila kundi lina mawe 18/3 = <<18/3=6>>6.
Idadi ya mawe ya manjano ni mawe 6 * rangi ya manjano = <<6*1=6>>6.
Tofauti kati ya idadi ya mawe ya buluu na manjano ni 13 - 6 = <<13-6=7>>7.