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4.Average
medium
The $average$ of $30$ numbers is $12 .$ The $average$ of the first $20$ of them is $11$ and that of the next $9$ is $10 .$ The last number is
A
$60$
B
$45$
C
$40$
D
$50$
Solution
Sum of all $30$ Number $=30 \times 12=360$
The last Number $=360-(20 \times 11+9 \times 10)$
$=360-(220+90)=50$
Standard 13
Quantitative Aptitude