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Notice that 7 2 = 8 × 9
We will show that the huge number H is divisible by both 8 and 9 and so it must be divisible by 72, and the remainder is zero.
First write H = k × 1 0 0 0 + 1 6 = 8 × ( 1 2 5 k + 2 ) to see thet H is divisible by 8.
The remainder when a number is divided by 9 equals its digital root. The digital root is the sum of all the digits of the number reduced modulo 9. The key insight is that the place values of the digits are irrelevant and so the digital root of H is equal to the digital root of
∑ i = 1 2 0 1 6 i = 1 0 0 8 × 2 0 1 7
Since 1008 is a multiple of 9, the digital root of H is zero, or in other words H is divisible by 9.
Now since H is a multiple of both 8 and 9, and they are relatively prime, H is divisible by 72 and the required remainder is 0