Question

Me ajudem

$\left \{ {{x+y= 20} \atop {3x + 4y= 72}} \right.$

Answer 1

x + y = 20 
x = 20 – y 

3x   +   4 y   = 72 
3 (20 – y) + 4y = 72 
 60-3y + 4y  = 72 
 -3y + 4y   =   72 – 60
       y = 12 

x = 20 – y. 
x = 20 – y 
x = 20 – 12 
x = 8 

S = {8,12}