Question

The average of $15$ numbers is $18$. The average of first $8$ is $19$ and that last $8$ is $17$, then the $8$th number is:

• A. $15$
• B. $16$
• C. $18$
• D. $20$

Answer 1

The correct option is C $18$

Average of 15 numbers=18

$\therefore$Sum of 15 numbers$=15\times 18=270$

Average of first 8 number is 19

$\therefore$Sum of first 8 number$=19\times 8=152$

Average of last 8 number=17

$\therefore$ Sum of 8 number$=8\times 17=136$

$\therefore$8th number is$=(152+136)-270$

$\Rightarrow 288-270=18$