Shawn has a budget of $50 per month to spend on bus fare. He made a table showing the total amount of bus money he will have left after the given number of rides.
Number of rides 0 9 18 27
Total amount of bus money left ($) 50 41 32 23

Which equation, in standard form, represents the relationship between the number of bus rides, x, and the amount of bus money Shawn will have left, y?

50x = y
x + y = 50
50y = x