In a class of 80 students, 60% are girls and the rest are boys.The average weight of boys is 5% more than that of girl. If the average weight of all student is 51 kg, then what is the average weight (in kg) of the boys?

Given, 

Total number of students = 80 

Number of girls = $$60\%\times\ 80\ =\ \frac{60}{100}\times\ 80=48$$

Number of boys = 80 - 48 = 32 

Average weight of boys = 5 % more than average weight of the girls

Average weight of the total students = 51

As we know, 

Average = $$\frac{sum\ of\ observation}{number\ of\ observation}$$

Let the average weight of girls be x.

then, Average weight of boys = $$\frac{105}{100}\times\ x=\frac{21x}{20}$$

According to question, 

$$\therefore\ \frac{\left(48\times\ x\right)+\frac{21}{20}x\times\ 32}{80}=51$$

by solving , we get 

x = 50 

Average weight of boys = 50$$\times\ \frac{21}{20}$$ = 52.5 kg

Hence, Option C is correct.