The ages of the students of a class form an AP whose common difference is 4 months. If the youngest is 8 years old and the sum of the ages of all the students is 168 years, find the number of students in the class.

Answer: A. 16

Work in months: a = 96, d = 4, sum = 168 × 12 = 2016.
S = n/2 (2a + (n - 1)d): 2016 = n/2 (192 + 4n - 4), so n² + 47n - 1008 = 0.
(n + 63)(n - 16) = 0, so n = 16 (n cannot be negative).

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