Solve for: x+40+x+80=100

Expression: $x+40+x+80=100$

Collect like terms

$2x+40+80=100$

Add the numbers

$2x+120=100$

Move the constant to the left-hand side and change its sign

$2x+120-100=0$

Subtract the numbers

$2x+20=0$

Divide both sides of the equation by $2$

$x+10=0$