Lizzy: Derek was 6 years old when he had three times as many dogs as cars. Ten years later, after selling some of his dogs and buying 210 more cars, the number of cars became twice the number of dogs. How many dogs does Derek have now if he had 90 dogs when he was six years old?.
Me: Hmmm, let me think. I think this is the detailed solution:
At six years old, with three times as many dogs as cars, Derek had 90 / 3 = 30 cars. Ten years later, after buying 210 more cars, Derek has 210 + 30 = 240 cars. Since the number of cars is twice as many as the number of dogs, Derek currently has 240 / 2 = 120 dogs. Final answer: 120.