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jmc

number theory senior

Problem

is the smallest positive multiple of 14 whose digits are all 1s and 0s. What is the quotient when is divided by 14?
Solution
Since must be divisible by , it must be divisible by and . Since it's divisible by , the last digit must be even, so the units digit must be . must also be divisible by . Let be the number obtained by taking and chopping off the last digit, . In order for to be divisible by , must be divisible by , and must also be made up of 's and 's. If has one digit, it must be (since ), which isn't divisible by . If has digits, it must be or , neither of which are divisible by . If has digits, it must be , , , or . Here we can use the divisibility rule for , 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 either. If has digits, we can check as we go along: if , then the divisibility rule reduces our checking to whether is divisible by , and we already know it's not. If , then the divisibility rule asks if is divisible by --and it is! So works. This means . We want the quotient .
Final answer
715