The average weight of 60 students in a class is 56.4 kg. If 12 students having average weight 59.5 kg leave the class and 15 students having average weight 54 kg join the class, then what is the average weight (in kg) of the students in the class (correct to one decimal place)?

Given, 

Average of 60 students weight is 56.4 kg

As we know, 

$$average\ =\ \frac{Sum\ of\ observation}{number\ \ of\ observation}$$

Sum of weights of 60 students = $$60\times\ 56.4=3384\ kg$$

• 12 students having average weight 59.5 kg leave the class (given)

Total weight of 12 students leaving the class 

= $$12\times\ 59.5=714\ kg$$

• 15 students having average weight 54 kg join the class

Total weight of 15 students who are joining the class 

= $$15\times\ 54=810\ kg$$

Total weight now ,

3384 - 714 + 810 = 3480

Total students now, 
60 - 12 + 25 = 63

Now, The average weight of class = $$\frac{3480}{63}=55.23\ kg$$

Hence, Option B is correct