The sum of two numbers is 27. One of the numbers is three more than the other.

x+y = 27 sum of two numbers is 27

x=y+3 One of the numbers is three more than the other

substitute x=y+3 into the first equation

(y+3) + y = 27

combine like terms

2y + 3 = 27

subtract 3 from each side

2y = 24

divide by 2

y = 12

x = y+3

x = 12+3

x = 15

the two numbers are 12 and 15