The Last Digit Only Matters

101 1 3 111 0 3 110 1 3 \large 1011_3 \qquad 1110_3 \qquad 1101_3

Which of the base 3 integers above is divisible by 3?

101 1 3 1011_3 111 0 3 1110_3 110 1 3 1101_3

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4 solutions

Relevant wiki: Integer Number Bases

111 0 3 = ( 3 3 × 1 ) + ( 3 2 × 1 ) + ( 3 1 × 1 ) + ( 3 0 × 0 ) = 39 1110_3=(3^3×1)+(3^2×1)+(3^1×1)+(3^0×0)=39 which is divisible by 3.

We can see that the other options are not divisible by 3.

We see that 1 0 10 10_{10} is divisible by 10 and 1 0 2 10_{2} is divisible by 2. Hence, we can say that a number who ends with 0 is divisible by the base in which it is written. So, 111 0 3 1110_3 is the only number to be divisible by 3

We can say it but not because it works for 10 and 2, because the rightmost digit of a number in base n is the division of said number by n. Hence if 0 is the remainder, it is divisible.

Hung Woei Neoh
May 18, 2016

101 1 3 = 3 3 + 3 1 + 3 0 3 3 + 3 1 + 3 0 3 = 3 2 + 1 + 1 3 111 0 3 = 3 3 + 3 2 + 3 1 3 3 + 3 2 + 3 1 3 = 3 2 + 3 1 + 1 110 1 3 = 3 3 + 3 2 + 3 0 3 3 + 3 2 + 3 0 3 = 3 2 + 3 + 1 3 1011_3 = 3^3 + 3^1 + 3^0\\ \dfrac{3^3 + 3^1 + 3^0}{3} = 3^2 + 1 + \dfrac{1}{3}\\ 1110_3 = 3^3 + 3^2 + 3^1\\ \dfrac{3^3 + 3^2 + 3^1}{3} = 3^2 + 3^1 + 1\\ 1101_3 = 3^3 + 3^2 + 3^0\\ \dfrac{3^3 + 3^2 + 3^0}{3} = 3^2 + 3 + \dfrac{1}{3}

Therefore, it is obvious that 111 0 3 \boxed{1110_3} is divisible by 3 3

Generally, for any positive number x x in base n n

x x is divisible by n n if the last digit is 0 0

Vijay Kumar
May 22, 2016

(a+b)%base=(a%base+b%base)%base
By this, we know that except the units place all the other positions always yield a multiple of base...in case if a rightmost bit is set, then 1 will be added, which when base>1 will always yield a remainder of 1...To say that the number is divisible by some number, we need to make the remainder 0...Title is the hint to get solution...(1110) is the solution here...

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