Question

The mean weight of 100 students in a class is 46 kg. The mean weight of boys is 50 and of girls is 40 kg. Therefore, the number of boys is:
A.50
B.60
C.70
D.65

Answer 1

60 is correct (60×50+40×40=4600)

Answer 2

$Number \: of \: students \: in \:a \:class = 100$

$Let \: number \:of \: boys = x$

$Let \: number \:of \:girls =( 100- x)$

$Mean \: weight \: of \: boys = 50$

$Mean \: weight \: of \: girlss = 40$

$Weight \: of \: 50 \: boys = 50x \:kg$

$Weight \: of \: 40 \: girls = 40(100-x)\:kg$

$Mean \: weight \: of \: students = 46 \:kg$

$\implies \frac{50x + 40(100-x)}{100} = 46$

$\implies 50x + 4000 - 40x = 4600$

$\implies 10x = 4600 - 4000$

$\implies 10x = 600$

$\implies x = \frac{600}{10}$

$\implies x = 60$

Therefore.,

$\red { Number \: boys } \green {= 60}$

$Option \: \pink { (B) } \: is \: correct$

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