Solve without a calculator– 2

Algebra Level 2

x = 120 ÷ 2.718 \large x = 120 \div 2.718

Round x x to the nearest integer.


The answer is 44.

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2 solutions

Brian Moehring
Nov 14, 2018

First we approximate the given quotient by 5 ! e = 120 e < 120 2.718 < 120 2.7 = 400 9 = 44. 4 ˉ \frac{5!}{e} = \frac{120}{e} < \frac{120}{2.718} < \frac{120}{2.7} = \frac{400}{9} = 44.\bar{4}

Using [ ] [\cdot] to denote the nearest integer function and ! n !n to denote the subfactorial, the above inequality implies 44 = ! 5 = [ 5 ! e ] [ 120 2.718 ] [ 44. 4 ˉ ] = 44 44 = {}!5 = \left[\frac{5!}{e}\right] \leq \left[\frac{120}{2.718}\right] \leq \left[44.\bar{4}\right] = 44 so the answer is 44 . \boxed{44}\text{.}


Notes: Obviously one can just solve this with a piece of paper and pencil [y'know, like before calculators?]. Due to the numbers chosen, however, it's interesting to give a solution which is more combinatorial in nature. Also, you might have noticed I didn't include the nontrivial step of evaluating ! 5 , !5\text{,} but anyone interested can evaluate it directly by using the recursive property ! n = n ! ( n 1 ) + ( 1 ) n ! 1 = 0 !n = n\cdot {}!(n-1) + (-1)^n \qquad !1 = 0

120 2.718 = 1200 27.18 \dfrac {120}{2.718} = \dfrac {1200}{27.18} which nearly equals. 1200 27 = 400 9 = 400 × 1 9 = 400 × 0.1111..... = 44.44444....... \dfrac {1200}{27} = \dfrac {400}{9} = 400\times \dfrac {1}{9} = 400\times 0.1111..... = 44.44444.......

A N S W E R : [ 44.44... ] = 44 ANSWER:[44.44...] = \boxed { 44 }

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