An undefined quantity which has had several proposed solutions:

Solution One: 0/0 = 0
Reason: Zero divided by anything is still zero.

Solution Two: 0/0 = 1
Reason: Anything divided by itself equals one.

Solution Three: 0/0 = infinity
Reason: Division by zero can result in infinity.

Solution Four: 0/0 = undefined
Reason: There is no answer!