Removing Floors - Part IV

Algebra Level 4

n 2 + n 3 + n 6 = n m \large \left \lfloor \dfrac n2 \right \rfloor + \left \lfloor \dfrac n3 \right \rfloor + \left \lfloor \dfrac n6 \right \rfloor =n-m

If n n and m m are two positive integers, then find the sum of all possible values of m m for the equation above.


Also check out the Part-I , Part-II and Part-III of the problem.


The answer is 3.

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1 solution

The following inequalities hold true:

0 n 6 n 6 > 1 0 n 3 n 3 > 1 0 n 2 n 2 > 1 Add ’em up, you get: 0 n 6 + n 3 + n 2 n > 3 Which comes to: n 0 n 6 + n 3 + n 2 > n 3 Since m is bounded above by 0 and below by 3, the only possibilities for m are 0,1,2: 6 6 + 6 3 + 6 2 = 6 = 6 0 5 6 + 5 3 + 5 2 = 3 = 5 2 4 6 + 4 3 + 4 2 = 3 = 4 1 Since all possibilities work for certain n , sum of all m = 0 + 1 + 2 = 3 0 \geq \lfloor {\frac{n}{6}} \rfloor - \frac{n}{6} > -1 \\ 0 \geq \lfloor {\frac{n}{3}} \rfloor - \frac{n}{3} > -1 \\ 0 \geq \lfloor {\frac{n}{2}} \rfloor - \frac{n}{2} > -1 \\ \text{Add 'em up, you get:} \\ 0 \geq \lfloor {\frac{n}{6}} \rfloor + \lfloor {\frac{n}{3}} \rfloor + \lfloor {\frac{n}{2}} \rfloor - n > -3 \\ \text{Which comes to:} \\ n-0 \geq \lfloor {\frac{n}{6}} \rfloor + \lfloor {\frac{n}{3}} \rfloor + \lfloor {\frac{n}{2}} \rfloor > n-3 \\ \text{Since } m \text { is bounded above by 0 and below by 3, the only possibilities for } m \text{ are 0,1,2:} \\ \lfloor {\frac{6}{6}} \rfloor + \lfloor {\frac{6}{3}} \rfloor + \lfloor {\frac{6}{2}} \rfloor = 6 = 6-0 \\ \lfloor {\frac{5}{6}} \rfloor + \lfloor {\frac{5}{3}} \rfloor + \lfloor {\frac{5}{2}} \rfloor = 3 = 5-2 \\ \lfloor {\frac{4}{6}} \rfloor + \lfloor {\frac{4}{3}} \rfloor + \lfloor {\frac{4}{2}} \rfloor = 3 = 4-1 \\ \text{Since all possibilities work for certain } n \text{, sum of all } m = 0+1+2=3

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