To find the least common multiple of the given numbers, you proceed as follow:
multiples of 5:
5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, ...
multiples of 25:
25, 50, 75, 100, ...
multiples of 15:
15, 30, 45, 60, 75, 90, ...
As you can notice, the least common multiple is the number 75, because it is the first number that appears in the multiples of 5, 25 and 15.