Question 258515
Let X = first number and Y = second number
"one number is 3 less than another" translates to
(i) X = Y  - 3
"their sum is 15" translates to
(ii) X + Y = 15
solve by substitution to get
(iii) Y - 3 + Y = 15
solving for Y we get
2Y - 3 = 15
2Y = 18
Y = 9
So,
X = 6
We have (X,Y) = (6,9)