The average 19

The average age of a class was 15 years. When 5 boys whose average age was 12 years 6 months were admitted in the class, the average was reduced by 6 months. How many students were there in the class originally?

Correct answer:

n =  20

Step-by-step explanation:

$$\begin{gathered} m_1=15 \\ {m}_2=12+6/12=\frac{25}{2}=12\frac{1}{2}=12.5 \\ 15-6/12=\frac{15n+5\cdot m_2}{n+5} \\ 14.5\cdot (n+5)=15n+5\cdot m_2 \\ 14.5\cdot (n+5)=15n+5\cdot 12.5 \\ 14.5\cdot (n+5)=15\cdot n+5\cdot 12.5 \\ 0.5n=10 \\ n=\frac{10}{0.5}=20 \\ n=20 \end{gathered}$$

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