Building Jack's Prism

Geometry Level 3

Jack has 222 222 lego cubes, each of side length 1. He puts them together to form a rectangular prism. If the perimeter of the base of the prism is 10 10 , what is the height of the prism?


The answer is 37.

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1 solution

Arron Kau Staff
May 13, 2014

Let a a and b b be the lengths of the rectangular base and h h be the height of the prism. a a , b b and h h must be positive integers. Then 2 a + 2 b = 10 a + b = 5 2a + 2b = 10 \Rightarrow a + b = 5 and a b h = 222 = 2 × 3 × 37 abh = 222 = 2\times 3 \times 37 . Furthermore, a a and b b are factors of 222 222 that sum to 5 5 , of which only 2 2 and 3 3 work. Thus, these are the lengths of the rectangular base, which allows us to calculate the height as h = 2 × 3 × 37 2 × 3 = 37 h = \frac{2\times 3 \times 37}{2 \times 3} = 37 .

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