They want you find the lowest positive integer number, which is divided by 3, 5 and 6 without remainder. Obviously, this integer must be a multiple of the product 3 and 5, i.e. 15, and as soon as you understand it, you will find the answer in 3 seconds: the number under the question is 30. This number is called the "Least Common Multiple" of numbers 3, 5 and 6, or LCM (3,5,6) for brevity. In this case it is simply the product of all prime numbers, that factor the numbers 3, 5 and 6.