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4.Average
medium
Average age of $15$ students of a class is $15\, years.$ Out of these the average age of $5$ students is $14 \,years$ and that of the other $9$ students is $16\, years.$ The age of the $15^{th}$ student is (in $years$)
A
$11$
B
$14$
C
$15$
D
$15 \frac{2}{7}$
Solution
Total age of $15$ students $=15 \times 15=225$ years…..$(1)$
Sum of ages of $5$ students $=5 \times 14=70$ years…..$(2)$
Sum of ages of $9$ students $=9 \times 16=144$ years…..$(3)$
$\therefore$ Age of $15^{th}$ student $=(1)-(2)-(3)=225-(70+144)=11$ years
Standard 13
Quantitative Aptitude