Question

The average of the first three numbers is double of the fourth number. if the average of all the four numbers is $12$. Find the $4th$ number.

• A. 16
• B. 48/7
• C. 20
• D. 18

Answer 1

The correct option is B 48/7

$\Rightarrow$ Let the four numbers are $x_1,x_2,x_3,x_4$

$\therefore$ $\frac{x_1+x_2+x_3+x_4}{4}=12$

$\therefore$ $x_1+x_2+x_3+x_4=48$

According to the question,

$\frac{x_1+x_2+x_3}{3}=2x_4$

$\therefore$ $x_1+x_2+x_3=6x_4$ ------- ( 1 )

$\therefore$ $x_1+x_2+x_3+x_4=48$

$\therefore$ $6x_4+x_4=48$ [ From ( 1 )]

$\therefore$ $7x_4=48$

$\therefore$ $x_4=\frac{48}{7}$