Binary Multiple of 14
$ T $ is the smallest positive multiple of 14 whose digits are all 1s and 0s. What is the quotient when $ T $ is divided by 14?
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
Since $ T $ must be divisible by $ 14 $, it must be divisible by $ 2 $ and $ 7 $. Since it's divisible by $ 2 $, the last digit must be even, so the units digit must be $ 0 $. $ T $ must also be divisible by $ 7 $. Let $ R $ be the number obtained by taking $ T $ and chopping off the last digit, $ 0 $. In order for $ T $ to be divisible by $ 7 $, $ R $ must be divisible by $ 7 $, and $ R $ must also be made up of $ 1 $'s and $ 0 $'s. If $ R $ has one digit, it must be $ 1 $ (since $ T\neq 0 $), which isn't divisible by $ 7 $. If $ R $ has $ 2 $ digits, it must be $ 10 $ or $ 11 $, neither of which are divisible by $ 7 $. If $ R $ has $ 3 $ digits, it must be $ 100 $, $ 101 $, $ 110 $, or $ 111 $. Here we can use the divisibility rule for $ 7 $, by chopping off the last digit, multiplying it by two, and subtracting it from the rest, to see that none of these values are divisible by $ 7 $ either. If $ R $ has $ 4 $ digits, we can check as we go along: if $ R=1000 $, then the divisibility rule reduces our checking to whether $ 100 $ is divisible by $ 7 $, and we already know it's not. If $ R=1001 $, then the divisibility rule asks if $ 98 $ is divisible by $ 7 $--and it is! So $ R=1001 $ works. This means $ T=10010 $. We want the quotient $ \frac{10010}{14}=\boxed{715} $.