Roberto is raising chickens for eggs. He buys 4 chickens for $20 each. They cost $1 in total a week to feed and each produces 3 eggs a week that will last forever. He used to buy 1 dozen eggs a week and spent $2 per dozen. After how many weeks will the chickens be cheaper than buying his eggs? Let's think first. Chain of thought:
The chickens cost $80 because 4 x 20 = 80. His chickens produce a dozen eggs a week because 4 x 3 = 12. Each week that he has chickens is $1 cheaper than buying eggs because 2 - 1 = 1. After 80 weeks his chickens will be paid off because 80 / 1 = 80. After 81 weeks the chickens will be cheaper than buying eggs because 80 + 1 = 81.
Therefore, the answer is 81.