first we list the multiples of each number.
multiples of 5 are 10,15,20,25,30,35,40,...
multiples of 6 are 12,18,24,30,36,42,48,...
multiples of 15 are 30,45,60,75,90,...
now, when you look at the list of multiples, you can see that 30 is the smallest number that appears in each list.
therefore, the least common multiple of...... 5,6 and 15 is 30.