Give the step-by-step reasoning process and then the final answer. Mrs. Thompson bought 3 chickens for $3 each. She also bought a bag of potatoes. Mrs. Thompson paid $15 in total. How much did the potatoes cost?
The 3 chickens amount to $3 x 3 = $9. Hence, the potatoes cost $15 - $9 = $6.
The final answer: 6.