The average weight of P, Q, R is 74 kg. S join the group the average weight decreased by 6 kg. Another person T join the group who is 3 kg heavier than S. The average weight of Q, R, S, and T is 64 kg. Find the weight of P of this group.

Let weights of P, Q, R be p, q, r. Average = 74, so p + q + r = 74 × 3 = 222 kg. When S joins, average decreases by 6 kg to 68 kg. This means S's weight is less than 74, and new total = 68 × 4 = 272 kg. So S = 272 - 222 = 50 kg. T is 3 kg heavier than S, so T = 53 kg. Average of Q, R, S, T is 64 kg, so q + r + 50 + 53 = 64 × 4 = 256. So q + r = 256 - 103 = 153 kg. From p + q + r = 222, p = 222 - 153 = 69 kg.