Question 25
There are five stacks, each containing five coins. Exactly one stack contains only counterfeit coins; the other four contain only genuine coins. All coins look alike. A genuine coin weighs 5 g and a counterfeit coin 4 g. Amy takes 1 coin from stack 1, 2 from stack 2, 3 from stack 3, 4 from stack 4, and 5 from stack 5. Together these coins weigh 71 g. Which stack contains the counterfeit coins? Enter its number.