Just a Simple Remainder problem

Compute the remainder when 4 96 4^{96} is divided by 6 6 .


The answer is 4.

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9 solutions

Tan Li Xuan
Nov 18, 2014

We can see that 4 2 4 ( m o d 6 ) 4^{2} \equiv 4 \pmod{6} So 4 96 4 48 4 24 4 12 4 6 4 3 ( m o d 6 ) 4^{96} \equiv 4^{48} \equiv 4^{24} \equiv 4^{12} \equiv 4^{6} \equiv 4^{3} \pmod{6} The question is equivalent to 64 ( m o d 6 ) 64 \pmod{6} which is just 4 \boxed{4}

Chew-Seong Cheong
Nov 18, 2014

4 96 1 6 48 ( m o d 6 ) 4 48 ( m o d 6 ) 4^{96} \equiv 16^{48} \pmod {6} \equiv 4^{48} \pmod {6}

1 6 24 ( m o d 6 ) 4 24 ( m o d 6 ) \quad \space \equiv 16^{24} \pmod {6} \equiv 4^{24} \pmod {6} (and it repeats)

1 6 12 ( m o d 6 ) 1 6 6 ( m o d 6 ) 1 6 3 ( m o d 6 ) \quad \space \equiv 16^{12} \pmod {6} \equiv 16^{6} \pmod {6} \equiv 16^{3} \pmod {6}

4 3 ( m o d 6 ) 64 ( m o d 6 ) 4 ( m o d 6 ) \quad \space \equiv 4^{3} \pmod {6} \equiv 64 \pmod {6} \equiv \boxed {4} \pmod {6}

Palash Som
Nov 17, 2014

we can simply solve this by forming a series as

4 , 16 , 64 , 256 , 1024 , 4096 ...........................................................................................

in each of the case remainder is 4 hence it can be clearly concluded that the remainder is 4.

It's not a solution but... I guessed 4. It's 50% chance. Possible reminders of any division to 6 are 1,2,3,4,5. Reminders 1,3,5 impossible because of 4^96 is even number. So my guess is correct.

Alex Gawkins - 6 years, 6 months ago

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Not a good method. You were lucky that you got it correct

Anuj Shikarkhane - 6 years, 6 months ago

In a free response question, you probably shouldn't do that. Besides, you should never eliminate the possibility of remainder being 0 unless it's pretty much certain you can prove.

tytan le nguyen - 6 years, 6 months ago

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You can prove that the remainder isn't 0. This is because 4 96 4^{96} isn't divisible by 6.

4 96 4^{96} is divisible by 2, but not divisible by 3.

The sum of the digits of powers of 4 go up in the sequence 3 n + 1 3n+1 , hence they are never divisible by 3.

Examples: Power of 4 (sum of digits), 4 (4), 16 (7), 64 (10), 256 (13), 1024 (10+2+4 = 16), 4096 (19), 16384 (22) ...

Sajid Mamun - 6 years, 5 months ago
Adarsh Kumar
Nov 18, 2014

4 ( 2 ) ( m o d 6 ) 4 2 ( 4 ) ( 2 ) ( m o d 6 ) 4 4 ( 4 2 ) ( 4 ) ( 2 ) ( m o d 6 ) 4 16 ( 2 ) ( m o d 6 ) 4 32 ( 2 ) ( m o d 6 ) 4 96 ( 8 ) ( 4 ) ( m o d 6 ) . 4\equiv(-2)\pmod{6}\\ 4^{2}\equiv(4)\equiv(-2)\pmod{6}\\ 4^{4}\equiv(4^{2})\equiv(4)\equiv(-2)\pmod{6}\\ \Rightarrow\ 4^{16}\equiv(-2)\pmod{6}\\ \Rightarrow\ 4^{32}\equiv(-2)\pmod{6}\\ \Rightarrow\ 4^{96}\equiv(-8)\equiv(4)\pmod{6}.

Rivaldo Tambahani
Dec 10, 2014

4/6 remainder =2==> (4^2)/6 remainder=4==> (4^3)/6 remainder=4==> (4^4)/6 remainder=4==> ... now we got the pattern. 2,4,4,4,...,4. only 4/6 that remainder 2.

so,(4^96)/6 remainder = 4

Mika Servi
Dec 10, 2014

96 divide 6 is 16 so it is finite has no remainder. the only no. left is 4 so my answer is 4. thats how i got it but i know it has no mathemaical basis

George Darroch
Nov 29, 2014

4^96=1^96=1 modulo 3. 4^96=0 modulo 2.

Therefore 4^96 is one more than a multiple of three and even.

Considering integers modulo 6, of the integers 0,1,2,3,4,5 only 4 satisfies the conditions therefore 4^96=4 modulo 6

Siddharth Singh
Nov 29, 2014

Just we can find 4^6 =4096. Then 4096/6 remainder=4

4^(96)=4^(16) * 4^(6). Now the remainder when divided by 6 is equal to the remainder in case of 4^(16). Remainder in case of 4^(16) is equal to remainder in case of 4^(4) which is again equal to the remainder in case of 4^(2). So, 16 i.e. 4^2=(6*2)+4. So 4 is the answer.

By using the Totient function and CRT we can compute mod 6 as mod 3 and mod 2. Any power of 4 is congruent to 0 mod 2 so 4^96 is 0 mod 2. For 4^96 mod 3 apply the Totient function. Since 3 is prime Φ(3) is just 3-1 or 2. so 4^2 is congruent to 1 mod 3. There fore (4^2)^48=1^48 mod 3 or 1 mod 3. So what is the smallest integer that is divisible by 2 and one more than a multiple of 3? 4.

Mr. Krabs - 4 months ago

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