I forgot the last digit of a 7- digit telephone number.

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I forgot the last digit of a 7- digit telephone number. If I randomly dial the final 3 digits after correctly dialing the first four, then what is the chance of dialing the correct number ?

1. 1/1001
2. 1/990
3. 1/999
4. 1/1000

Correct Option: D

it is given that last 3 digits are randomly dialed
Then, each of the digit can be selected out of 10 digits in 10 ways. Hence, required probability
= 1/(10)³ = 1/1000