Question

calcule as sistema
x+y=20
x-y=6

Answer 1

Ola Viera 

x + y = 20 
x - y = 6

adição

2x = 26
x = 26/2 = 13

13 + y = 20
y = 20 - 13 = 7 

Answer 2

x=20-y

(20-y)-y=6
-2y=6-20
-2y=-14
y=-14/-2
y=7

x=20-7
x=13