Last two digits

Find the last 2 digits of the product 7 × 19 × 31 × 43 × × 1999 7\times 19\times 31\times 43\times \cdots \times 1999


The answer is 75.

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1 solution

Marco Brezzi
Aug 10, 2017

N = k = 0 166 7 + 12 k N=\displaystyle\prod_{k=0}^{166} 7+12k

We will consider N N mod 4 4 and mod 25 25 separately

N = k = 0 166 7 + 12 k k = 0 166 1 + 0 ( 1 ) 167 1 3 m o d 4 \begin{aligned} N &=\displaystyle\prod_{k=0}^{166} \mathbin{\color{#D61F06} 7}+\mathbin{\color{#3D99F6} 12k}\\ & \equiv \displaystyle\prod_{k=0}^{166} \mathbin{\color{#D61F06} -1} + \mathbin{\color{#3D99F6} 0}\\ & \equiv (-1)^{167} \equiv -1 \equiv 3 \mod 4 \end{aligned}

The product contains the term 7 + 12 14 7+12\cdot 14 and we have

7 + 12 14 = 7 ( 1 + 12 2 ) = 7 25 0 m o d 25 7+12\cdot 14=7(1+12\cdot 2)=7\cdot 25\equiv 0 \mod 25

Which implies N 0 m o d 25 N\equiv 0 \mod 25

We now have the following system of congruences

{ N 3 m o d 4 N 0 m o d 25 \begin{cases} N\equiv 3 \mod 4\\ N\equiv 0 \mod 25 \end{cases}

And by the CRT we have only one solution mod 100 100 , namely N 75 m o d 100 N\equiv \boxed{75}\mod 100

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