How Do I Remove These Floors?

Find the number of solutions to the equation n 2 + n 3 + n 6 = n 1 \large \left \lfloor \dfrac n2 \right \rfloor + \left \lfloor \dfrac n3 \right \rfloor + \left \lfloor \dfrac n6 \right \rfloor =n-1

where n n is a natural number such that 1 n 100 1\leq n \leq 100 .


Be sure to check out Part-I , and Part-2 of this problem.


The answer is 68.

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4 solutions

Peter Macgregor
Apr 7, 2016

Each integer n can be written as n = 6 k + p n=6k+p where k and p are integers and 0 p 5 0\leq p \leq 5 .

Now for each value of p the left hand side of the equation can be evaluated term by term to get

For p = 0, n = 6 k n=6k and the left hand side becomes

( 3 k ) + ( 2 k ) + ( k ) = 6 k n 1 (3k)+(2k)+(k)=6k\neq n-1 and so n = 6 k n=6k is NOT a solution of the equation.

For p = 1, n = 6 k + 1 n=6k+1 and the left hand side becomes

( 3 k ) + ( 2 k ) + ( k ) = 6 k = n 1 (3k)+(2k)+(k)=6k= n-1 and so n = 6 k + 1 n=6k+1 is a solution of the equation.

For p = 2, n = 6 k + 2 n=6k+2 and the left hand side becomes

( 3 k + 1 ) + ( 2 k ) + ( k ) = 6 k + 1 = n 1 (3k+1)+(2k)+(k)=6k+1= n-1 and so n = 6 k + 2 n=6k+2 is a solution of the equation.

For p = 3, n = 6 k + 3 n=6k+3 and the left hand side becomes

( 3 k + 1 ) + ( 2 k + 1 ) + ( k ) = 6 k + 2 = n 1 (3k+1)+(2k+1)+(k)=6k+2 = n-1 and so n = 6 k + 3 n=6k+3 is a solution of the equation.

For p = 4, n = 6 k + 4 n=6k+4 and the left hand side becomes

( 3 k + 2 ) + ( 2 k + 1 ) + ( k ) = 6 k + 3 = n 1 (3k+2)+(2k+1)+(k)=6k+3 = n-1 and so n = 6 k + 4 n=6k+4 is a solution of the equation.

For p = 5, n = 6 k + 5 n=6k+5 and the left hand side becomes

( 3 k + 2 ) + ( 2 k + 1 ) + ( k ) = 6 k + 3 n 1 (3k+2)+(2k+1)+(k)=6k+3 \neq n-1 and so n = 6 k + 5 n=6k+5 is NOT a solution of the equation.

By examining the above cases we see that the equation fails to have a solution whenever n is a multiple of 6 or one less than a multiple of six. So for n between 1 and 100 the number of failures is

2 × 100 6 = 32 2 \times \lfloor \frac{100}{6} \rfloor = 32

and so the number of successes is

100 32 = 68 100-32=\boxed{68}

Thank you, long solution but really simple

Hải Trung Lê - 5 years, 2 months ago

Nice solution!

milind prabhu - 5 years, 2 months ago

Define, f ( n ) = n 2 + n 3 + n 6 g ( n ) = f ( n ) ( n 1 ) f ( n + 6 ) = f ( n ) + 6 g ( n + 6 ) = g ( n ) f(n)=\left \lfloor\frac{n}{2}\right \rfloor+\left \lfloor\frac{n}{3}\right \rfloor+\left \lfloor\frac{n}{6}\right \rfloor\\g(n)=f(n)-(n-1)\\\implies f(n+6)=f(n)+6\\\implies g(n+6)=g(n)

So, if g ( n ) = 0 g ( n + 6 k ) = 0 k Z g(n)=0\implies g(n+6k)=0 \quad \forall k \in \mathbb{Z} .

Now, note that g ( 1 ) = g ( 2 ) = g ( 3 ) = g ( 4 ) = 0 g ( 5 ) 0 g ( 6 ) 0 g(1)=g(2)=g(3)=g(4)=0 \quad g(5)\neq0 \quad g(6)\neq0 g ( n ) = 0 n k ( m o d 6 ) k = 1 , 2 , 3 , 4 g(n)=0\implies n \equiv k \pmod6\quad k=1,2,3,4

Hence, the number of required solutions is 68 \boxed{68} .

Essentially the same idea as Peter's. I must appreciate the way your solution has been written. Compact and clear!

milind prabhu - 5 years, 2 months ago

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well. f (n + 6) is not k.

Am Kemplin - 3 weeks, 4 days ago

We know that every integer can be written in form of 6 κ + ψ 6\kappa + \psi . Where

κ N \kappa \in N and

0 ψ 5 0\leq \psi \leq 5 , ψ \psi is also a natural number.

Now

s u c h κ , ψ = 0 , 5 \forall such \kappa , \psi = 0,5 does not satisfies. Hence 32 solutions are to be emitted from o n e one hundred.

Hence answer is 68 \boxed{68} .

Andreas Wendler
Apr 7, 2016

Via induction one can recognize that among the 100 possible values for n always 2 in equidistance must be removed resp.:

n k = 5 + 6 k n_{k}=5+6k ; n k + 1 = 6 ( k + 1 ) n_{k+1}=6(k+1) with 0<=k<=15)

As result there are 100 - 2*16 =68 solutions.

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