A machine makes 20 packets in the first hour and 5 more packets each next hour. How many packets are made in the 10th hour?

Correct answer: 65 packets

The governing concept is the nth term of an arithmetic progression. The hourly production is 20, 25, 30, and so on, because the same difference of 5 packets is added each hour. Thus the first term is a = 20, the common difference is d = 5, and the required term number is n = 10. Applying aₙ = a + (n − 1)d gives a₁₀ = 20 + (10 − 1) × 5 = 20 + 45 = 65 packets. Therefore option C is correct. The tenth hour is reached after nine increases from the first hour, not ten. Options A and B count too few increases, while D counts too many. Since the question asks for production in one hour, a term formula, not a sum formula, is required.