In a class, there are 39 students and their average weight is 51 kg. If we include the weight of the teacher, then the average weight becomes 51.2 kg. What is the weight of the teacher?

59 kg

Sum of weight of 39 students = 39 × 51 = 1989 kg

Sum of weight of 39 students & 1 teacher = 40 × 51.2 = 2048 kg

Weight of Teacher = 2048 - 1989 = 59 kg