Question

80-x-x=0 afla necunoscuta

Answer 1

80  =  x + x 
80 = 2x 
x =80 :2 
x = 40 

Answer 2

$80-x-x=0 \\ 80-2x=0 \\ -2x=0-80 \\ -2x=-80 \\ x=-80:(-2) \\ x=40.$