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Bob constructs a rectangular building by many $1\times1\times1$ cubes. The volume of the rectangular building is $210$. What is the least sum of all edges of the rectangular building? 
Overseas Competition->Knowledge Point->Combinatorics->Combinatorics Involving Extreme Values->Problems of Extreme Value with Fixed Products
Given the product, the smaller the difference, the smaller the sum.  $210=2\times3\times5\times7$, so the length, the width, and the height are $5,$ $6,$ and $7$. $(5+6+7)\times4=72.$ 