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4.Average
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The average age of a graph of $9$ students is $20\, years.$ When $6$ more students joined the group, the average age falls by $2\, years.$ Then the average age of new students is
A
$20$
B
$18$
C
$19$
D
$15$
Solution
Average age of new student $=$
$\frac{Old\, Number \,of \,student \,\times \,Old\, average \,of \,students \,- \,New \,average \,\times \,New \,number\, of \,students}{Number \,of \,New \,students}$
$=\frac{9 \times 20-18 \times 15}{6}=\frac{180-270}{6}=\frac{90}{6}=15$ years
Standard 13
Quantitative Aptitude
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