Least common multiple of a set of numbers is the lowest number that all the numbers in the set will divide into exactly. To find it you need to analyse the numbers in the set to see if there are any common factors. That means, is there a number that divides exactly into more than one number in the set? In the question, 3 divides exactly into 3 and 15. So we ignore 3 because we now know that if 15 goes into the lowest common multiple, so must 3. There is no common factor between 15 and 17, because 17 isn't divisible by 3 or 5, which make up the number 15. Therefore we just multiply 17 by 15 = 255, and this is the LCM.