The average of weight of 8 students is 24.3 kg., out of which the average of first two is 18.5 and that of next three is 21.2. If the weight of the sixth student is 3 kg. less than the weight of the seventh student and 8 less than eighth student, what is the weight of the sixth student?

The total weight of all 8 students is 8 × 24.3 = 194.4 kg. The first 5 students weigh 2 × 18.5 + 3 × 21.2 = 100.6 kg. Therefore, the last 3 students together weigh 194.4 - 100.6 = 93.8 kg. Let the 6th student's weight be x. Then the 7th weighs x + 3 and the 8th weighs x + 8. Solving x + (x+3) + (x+8) = 93.8 gives x = 27.6 kg.