Let the variable s represent the amount Trey saved before mowing lawns.
Trey earns s dollars. Then he earns an additional $40. So far, he earns (s+40).
Then this amount is doubled: 2(s+40)
At the end, he has $234. So the equation is
2(s+40) = 234
2s + 80 = 234
2s = 154
s = 77
An arithmetic approach (solving using addition) could have the equation as
(s+40) + (s+40) = 234
s + 40 + s + 40 = 234
2s = 154
s = 77
