The average weight of a class of 39 students is 40 kg. When a new student is admitted to the class, the average decreases by 200 g

Average weight of 39 students = 40 kg

Sum of the weights of 39 students $=(40 \times 39) \mathrm{kg}=1560 \mathrm{~kg}$

Decrease in the average when new student is admitted in the class = 200 g = 0.2 kg
∴ New average weight = (40 -">−- 0.2) kg = 39.8 kg
Now,
Let the weight of the new student be x kg.
Thus, we have:

$\frac{\text { Sum of the weights of } 39 \text { students }+x}{40}=39.8$

$\Rightarrow \frac{1560+x}{40}=39.8$

$\Rightarrow 1560+x=1592$

$\Rightarrow x=32$

Therefore, the weight of the new student is 32 kg.