The average weight of 20 students in a class is 55 kg. If the weight of the class teacher is included, the average is increased by 0.5 kg. The weight of the class teacher is:

Correct option is C

Total weight of 20 students =55 kg×20=1100 kg

​New average weight=55 kg+0.5 kg=55.5 kg​

$$\text{Number of people now}=20\text{ students}+1\text{ teacher}=21$$

$$Total\; weight\; of\; 21\; people=55.5\; kg\times 21=1165.5\; kg$$

$$Weight\; of\; teacher\; =\; 1165.5\; -1100\; =\; 65.5\; kg$$

Total weight of 21 people=55.5 kg×21=1165.5 kg\text{Total weight of 21 people} = 55.5 \text{ kg} \times 21 = 1165.5 \text{ kg