If Mary spent half of her allowance the equation will begin with 1/2x or 0.5x where x is her allowance and 0.5 is half of her allowance spent. Mary also earned 7 dollars by washing the car so this will be added to her allowance spent.
The problem says that after Mary has spent half of her allowance at the movies she was left with 18 dollars so that means that the equation must be equal to 18.
The equation you want to set up is
0.5x+7=18
Where, 18-7=11
0.5x=11
11/.5 = x
x = $22
Thus, Mary has a weekly allowance of 22 dollars.
