Question 927136
Let the two numbers be x and y
Let the smaller number = x
Let the larger number = y

Sum of two numbers = 18
x + y = 18  ...eqn. (1)

The larger number is three times the smaller.
y = 3x  ... eqn. (2)

Subst. 3x for y in eqn. (1)
x + 3x = 18
4x = 18
x = 4.5

Subst. 4.5 for x in eqn. (2)
y = 3(4.5)
y = 13.5

(x, y) = (4.5, 13.5)