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### Question

The ratio of surface area to the volume of a cuboids is 9 : 1, and the sum of reciprocals of two edges is 74, what is the reciprocal of the third edge?

**A**

28 .

**B**

30 .

**C**

26 .

**D**

32 .

**Soln.**

**Ans: a**

The well-known relation for a cuboids is $1/V = 2/S(1/a + 1/b + 1/c)$, which can be re-arranged to get $1/c = 1/2(S/V) - (1/a + 1/b)$ = $1/2(74) - 9$ = 28.