The floor is a function 1

Algebra Level 5

FInd the sum of all positive integers x x satisfying the equation

x 20 + x 17 = 2017 \left \lfloor \frac{x}{20} \right \rfloor + \left \lfloor \frac{x}{17} \right \rfloor = 2017

This problem is part of the set " Xenophobia "


The answer is 129801.

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2 solutions

Rindell Mabunga
Jun 27, 2017

Let y y be a real number. Since y \lfloor y \rfloor is the integer less than or equal to y y , then y 1 < y y y - 1 < \lfloor y \rfloor \leq y .

In the same manner, x 20 + x 17 2 < x 20 + x 17 x 20 + x 17 \frac{x}{20} + \frac{x}{17} - 2 < \left \lfloor \frac{x}{20} \right \rfloor + \left \lfloor \frac{x}{17} \right \rfloor \leq \frac{x}{20} + \frac{x}{17}

x 20 + x 17 2 < 2017 x 20 + x 17 \frac{x}{20} + \frac{x}{17} - 2 < 2017 \leq \frac{x}{20} + \frac{x}{17}

37 x 340 2 < 2017 37 x 340 \frac{37x}{340} - 2 < 2017 \leq \frac{37x}{340}

18534.59 2017 × 340 37 x < 2019 × 340 37 18552.97 18534.59 \approx 2017 \times \frac{340}{37} \leq x < 2019 \times \frac{340}{37} \approx 18552.97

18535 x 18552 18535 \leq x \leq 18552

Since 18535 20 + 18535 17 = 926 + 1090 = 2016 \left \lfloor \frac{18535}{20} \right \rfloor + \left \lfloor \frac{18535}{17} \right \rfloor = 926 + 1090 = 2016 , we need to look at the next two integers which is a multiple of 20 20 or 17 17 which is 18540 18540 and 18547 18547 . Therefore the integral values of x that we need are 18540 18540 , 18541 18541 , \ldots , 18546 18546 .

18540 + 18541 + + 18546 = 129801 18540 + 18541 + \ldots + 18546 = \boxed{129801}

Sir apparently someone named "Rindell" shared your problem in our math group. Beware, he might be an impostor :p

Manuel Kahayon - 3 years, 11 months ago

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He's my alter ego xD

Rindell Mabunga - 3 years, 11 months ago
Ravneet Singh
Jun 27, 2017

For start, let us take the equation as simple fraction x 20 + x 17 = 2017 \dfrac{x}{20} + \dfrac{x}{17} = 2017

upon solving we get x 18 , 534.595 x \approx 18,534.595

if we put this value in original equation we get 18 , 534.595 20 + 18 , 534.595 17 = 2016 \left \lfloor \dfrac{18,534.595}{20} \right \rfloor + \left \lfloor \dfrac{18,534.595}{17} \right \rfloor = 2016

which is one less than what is required. So, for increasing this value by one, we need a nearest number which is multiple of either 20 or 17, which in this case is 18,540. Now all values from 18,540 to 18,546 will satisfy the given equation as 18,547 is a multiple of 17, thereby will increase the value to 2018.

Finally sum of all values = 18 , 540 + 18 , 541 + + 18 , 546 = 129801 = 18,540 + 18,541 +\cdots + 18,546 = \boxed{129801}

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