If a person has 20 to spend on rides in a park.

Hello I have question i hope somebody will help me. If a person has 20 to spend on rides in a park. The first ride gives him 20 total utility and the second gives him 30 TU and he decides to go on 1 ride. not to mention each ride costs 10 dollars. is the marginal utility of the second ride is zero since he only decide to ride the first?

Thank you