Question

Shelly, Jess, and Andrea are all working at an amusement park for the summer. The manager finds out that one of them left the key in the door to the security office. When he asked them who did it, they replied:

Shelly: Jess did it.
Jess: I didn’t do it.
Andrea: Jess did not do it.

The manager believed Jess. Use an indirect proof to show that Jess did not leave the key in the security office door.

Hint: Begin by assuming Jess did leave the key in the door.