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.

Answer 1

Is it something to do with what Andrea said?

Answer 2

No I'd think not.

Answer 3

I think:" shelly is the only one accused someone else I would have has to say shelly."

Answer 4

i found a link related to indirect proofs: http://www.saskschools.ca/curr_content/mathc30/unit6/Lesson6.htm

Answer 5

This is what I am going to submit to my teacher.

Answer 6

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

Prove: Jess did not do it.

Since two of the three statements contradict the assumption, the assumption is false. Because the assumption was proven false, it's opposite must be true.

Therefore, Jess did not do it.

Answer 7

I think you are right :D

Answer 8

Thank you for your help :D

Answer 9

I think I would say:
assume Jess did it

then I would expect
Shelly to say "Jess did it"
Andrea to say "Jess did it"
Jess to say "I did not do it"

because Andrea actually said, "Jess did not do it", the assumption is wrong, and we conclude Jess did not do it.