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Faulty Streetlights is a puzzle in Professor Layton and the Last Specter.

Puzzle

US Version

This quiet road is lit by 10 street-lights. Deep in the dead of the night, the lights are blown by a power surge and all the lights are replaced.

Unfortunately, the new bulbs are poor quality--one breaks every two hours. Finding it hard to keep up, the unlucky repairman only manages to bring a replacement bulb every three hours.

How many bulbs will still be lit after 12 hours?

UK Version

This quiet road is lit by ten streetlights. Deep in the dead of night, the lights are blown by a power surge and all the lights are replaced.

Unfortunately, the new bulbs are poor quality, and one breaks every two hours. Finding it hard to keep up, the unlucky repairman only manages to bring a replacement bulb every three hours.

How many bulbs will still be lit after 12 hours?

Hints

Click a Tab to reveal the Hint.

The key is to think about the state of the lights after 12 hours.

US Version

To begin with, all 10 lights were shining brightly in the night.

Now try to imagine the scene 12 hours later.

UK Version

To begin with, all ten lights were shining brightly in the night.

Now try to imagine the scene 12 hours later.

US Version

You don't need to do any math to answer this puzzle. If you just think about what streetlights are used for, the answer should be obvious.

UK Version

You don't need to do any maths to answer this puzzle. Just think about what streetlights are used for and the answer should be obvious.

If you imagine that the power surge happened at midnight and the new bulbs were fitted and working shortly afterwards, what time will it be 12 hours later?

Will there be any need for lights then?

Solution

Incorrect

US Version

With a bit more thought, you might get a flash of inspiration.

UK Version

With a bit more thought, you might get a flash of inspiration.

Correct

US Version

Very bright of you!

After 12 hours, it will no longer be dark outside, so all of the streetlights will be turned off.

UK Version

What a bright spark!