Nine members of a family are standing in a row. Their average age is 70. The average age of the first four members in the row is 64 and the average age of the last seven members in the row is 80. What is the average age of the third and fourth member in the row?

- a)78
- b)86
- c)93
- d)89
- e)87

Correct answer is option 'C'. Can you explain this answer?

2 Answers

Veera Reddy Konabhai
answered
6 days ago

Consider first two members as set A, third person as B, fourth person as C and remaining five membes as set D.

given (A+B+C+D)/9 = 70 => A+B+C+D = 630

and (A+B+C)/4 = 64 => A+B+C = 256 ----> smt1

and (B+C+D)/7 = 80 => B+C+D = 560 ----->smt2

now add smt1 and smt2 that gives us A+B+C+B+C+D = 816 and given A+B+C+D is 630 subitting in previous equation we get B+C+630 = 816 and solving gives us B+C = 186. now we want avg of B,C that is (B+C)/2 = 186/2 = 93

given (A+B+C+D)/9 = 70 => A+B+C+D = 630

and (A+B+C)/4 = 64 => A+B+C = 256 ----> smt1

and (B+C+D)/7 = 80 => B+C+D = 560 ----->smt2

now add smt1 and smt2 that gives us A+B+C+B+C+D = 816 and given A+B+C+D is 630 subitting in previous equation we get B+C+630 = 816 and solving gives us B+C = 186. now we want avg of B,C that is (B+C)/2 = 186/2 = 93

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