中二數學消元法

x-y=4........(1)
(x+y)/2=7.........(2)
x=4+y.........(3)
(3)→(1)
((4+y)+y)/2=7
4+2y=14
2y=10
y=5
(y=5)→(1)
x-5=4
x=9

let the two number be x and x-4

{x + (x-4)} 2 = 7
2x-4 = 14
x = 9

so, the two numbers are 9 and 5