You push two buttons into the “on” position and wait for a few minutes. You then push one of those two buttons into the “off” position and go across to the machining hall.
If the motor is on, it must be governed by the one button you left in the “on” position.
Feel the housing of the motor. If it is warm, then the machine has been running and it must be governed by the button you pushed “off” just before you headed over. If it is cold, then it has not been running at all and it must be governed by the final button that was never in the “on” position.
The Bursar was arrested. The police knew he was lying as the bank is closed on Sundays.
You take a shell from the box labelled as a mix of both. Since the “mixed” label definitely wrong, you know that if you unwrap a snail shell, the whole box must be snail shells and vice versa.
Say you did pick out a snail shell, and thus know the box labelled mixed is, in fact, full of snail shells. The box labelled oyster shells cannot have only oyster shells inside, as we know the label is wrong. Since the snail box is already accounted for, the box labelled oysters must actually be mixed. This means that the box labelled snail shells must, therefore, contain oyster shells.