Question

Average of $25$ number is $18$. Average of first 12 number is $14$ and last $12$ number is $17$. Then what is the $13th$ number?

• A. 72
• B. 78
• C. 85
• D. 28

Answer 1

Answer: (B)

78

Average of $25$ numbers $=18$

$\Rightarrow$ Sum ($25$ numbers)$=25\times 18\Rightarrow 450$

average of first $12$ numbers $=14$

$\Rightarrow$ Sum (first $12$ numbers)=$12\times 14\Rightarrow 168$

average of last $12$ numbers $=17$

$\Rightarrow$ Sum ($25$ numebers)=Sum (first $12$)+sum(last $12$)+${13}^{th}$ number

$450=168+204+{13}^{th}$ number

$450=372+{13}^{th}$ number

$\Rightarrow {13}^{th}$ number $=450-370=78$