| In a class of 100 students there are 70 boys whose average marks in a subject are 75. |
| If the average marks of the complete class is 72, then what is the average of the girls? |
A) 73
B) 65
C) 68
D) 74
Correct Answer: B
Solution :
[b]:\[\overline{x}=72\] (Number of boys)\[{{n}_{1}}=70\] (Number of girls)\[{{n}_{2}}=30\] \[{{\overline{x}}_{1}}=75;{{\overline{x}}_{2}}=?\] \[\overline{x}=\frac{{{n}_{1}}{{\overline{x}}_{1}}+{{n}_{2}}{{\overline{x}}_{2}}}{{{n}_{1}}+{{n}_{2}}}\Rightarrow 72=\frac{70\times 75+{{\overline{x}}_{2}}\times 30}{70+30}\] \[\Rightarrow \]\[{{\overline{x}}_{2}}=65\].
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