We can write an equation:
y = 7x + 25
Where 'y' is the total cost and 'x' is the number of cars she washes.
Plug in 65 for 'y' and solve for 'x':
65 = 7x + 25
Subtract 25 to both sides:
40 = 7x
Divide 7 to both sides:
x = 5.71
I guess we should round to the nearest whole number, so we round to 6 cars.