The total weight of all 9 people is the sum of the weights of the first 5 and the last 5, minus the weight of the 5th person (who is counted twice). If the total weight of all 9 is 9 * 50 = 450 kg, the combined weight of the first 5 is 5 * 45 = 225 kg, and the combined weight of the last 5 is 5 * 55 = 275 kg. The sum of these two groups is 225 + 275 = 500 kg. This excess (500 - 450 = 50 kg) represents the weight of the 5th person, as they were included in both group calculations.