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

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

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

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: Andrea did it.
Jess: Shelly did not do it.
Andrea:  I didn’t do it.
 
The manager believed Andrea. Use an indirect proof to show that Andrea did not leave the key in the security office door.
 
Hint: Begin by assuming Andrea did leave the key in the door.

anonymous · Answer

I am kinda confused here, even with the hint, it's a little confusing.