A teacher writes a positive integer N on the board and asks her 20 students which numbers divide it.

Student 1 says 1, student 2 says 2, ....., student 20 says 20. Only 2 students make mistakes, and they are consecutive. Which 2 students make the mistakes and what is the smallest possible value of N?

Well, the mistake can't be 1-10, and it can't be a number that can be written as n=ab, a>1, b>1, (a,b)=1, and so this leaves the only two consecutive possibilities as 16 and 17. My problem is in finding the smallest possible N. Do you just multiply all of the remaining numbers that aren't multiples of each other? How can I show it is the smallest?