2016 gone wrong

What is the remainder when you divide the product of the last three digits of 201 6 2017 2016^{2017} with 320?


The answer is 240.

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1 solution

Harsh Khatri
Feb 26, 2016

( 2016 ) 2017 1 6 2017 1 6 ( 5 × ϕ ( 1 0 3 ) + 17 ) 2 4 × 17 ( m o d 1 0 3 ) (2016)^{2017} \equiv 16^{2017} \equiv 16^{\big(5 \times\phi(10^3) + 17\big) } \equiv 2^{4\times 17} \pmod{10^3}

Now comes the messy part.

2 7 3 ( m o d 5 3 ) 2^7 \equiv 3 \pmod{5^3}

2 65 = 2 2 ( 2 7 ) 9 4 ( 3 9 ) 4 ( 3 5 ) ( 3 4 ) 4 ( 7 ) ( 81 ) 107 ( m o d 5 3 ) 2^{65} = 2^2\cdot (2^{7})^{9} \equiv 4(3^{9}) \equiv 4(3^{5})(3^{4}) \equiv 4(-7)(81) \equiv 107 \pmod{5^3}

Now we multiply the whole congruence with 2 3 2^3 .

2 65 + 3 107 × 8 = 856 ( m o d 5 3 × 2 3 ) 2^{65+3} \equiv 107\times 8 = 856 \pmod{5^3 \times 2^3}

Hence, the last 3 3 digits of ( 2016 ) 2017 \displaystyle (2016)^{2017} are 856.

Their product when divided by 320 320 gives 240 \boxed{240} .

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