Discussion
There are five rooms in my house - a hall, kitchen, study room, bedroom and dining room. There is one light in each room. Whenever the kitchen light is on, the study room light is off. Whenever the dining room light is on, the kitchen light is also on. Whenever the study room light is on, the hall light has to be on. How many lights, at the most, can be on simultaneously ?
Answer: B
At most 4 lights can be on together.
The rules are: kitchen on means study off; dining on means kitchen on; study on means hall on. The kitchen and study lights can never be on together, so all 5 is impossible.
Switch on the kitchen, dining room, hall and bedroom lights, with the study off. Every rule holds: the kitchen is on, so the study is off; the dining room is on and so is the kitchen; the study rule does not apply.
Choosing the study instead forces the kitchen off, which forces the dining room off, leaving at most study, hall and bedroom: only 3. So 4 is the maximum, and 3 and 2 are below it.
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