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Is the key to the water meter well easy to open?

What are the chances that the key won't open?

The probability of getting an unlockable key is 1/3.

The probability of getting a key that can unlock, but can't, is

1/3* 1/2+ 1/3* 1/2= 1/3.

So the total probability of not unlocking is 2/3.

The total probability of unlocking is 1- 1/3=2/3.

I hope I can help you.