12) C

Total time taken by first group = 54 x 20 = 1080s

We know number of people in second group = ‘P’

Mean time of second group = ‘T’

Total number of people = 20 + P

Total time taken by second group = T x P

Total time taken by both groups = 1080 + (T x P)

In order to get total mean time (56), we need to divide total amount of time by total number of people.

56 = (1080 + TP) ÷ (20 + P)

56 (20 + P) = 1080 + TP

1120 + 56P = 1080 + TP

1120 – 1080 = TP – 56P

40 = P (T – 56)

P = 40 / (T – 56)

Hence, the answer is C.

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