The average of first three numbers is double of the fourth number. If the average of all the four numbers is 12, find the 4th number.
Options:
A . 20
B . $18/7$
C . 16
D . $48/7$
Answer: Option D Answer: (d)$\text"a+b+c"/3$ =2d⇒ a + b + c = 6d ...(i) Also, $\text"a+b+c+d"/4$ = 12 ⇒ a + b + c + d = 48 ⇒ 6d + d = 48 ⇒ 7d = 48 ⇒ d = $48/7$
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