We are given that the sum of two numbers is 24. If "x" and "y" are the two numbers then we have that:

We are also given that one number is three times the other, this is expressed as:

Now, we substitute the value of "x" from the second equation in the first equation:

Now, we add like terms:

Now, we divide both sides by 4:

Therefore, the first number is 6. Now, we substitute the value of "y" in the second equation:

Therefore, the other number is 18.