A field soil holds a pesticide at 64 mg per kg. Its degradation is first order with half-life 2.5 years. A safe limit of 2 mg per kg is set. After how long is the limit first reached?

Answer: B

Concentration falls

64 \to 32 \to 16 \to 8 \to 4 \to 2

, which is 5 half-lives since

64/ 2^{5} = 2

. Time

= 5 \times 2.5 = 12.5

years. A linear calculation giving 80 years is wrong.