# Moderate Probability Solved QuestionAptitude Discussion

 Q. I forgot the last digit of a 7-digit telephone number. If 1 randomly dial the final 3 digits after correctly dialling the first four, then what is the chance of dialling the correct number?
 ✖ A. 1/1001 ✔ B. 1/1000 ✖ C. 1/999 ✖ D. 1/990

Solution:
Option(B) is correct

It is given that last three digits are randomly dialled. then each of the digit can be selected out of 10 digits in 10 ways.

Hence required probability

$=\left(\dfrac{1}{10}\right)^3$

$=\dfrac{1}{1000}$

## (8) Comment(s)

RGSL
()

Why didn't you consider the the repetition of the numbers in the last three places lik 887, 999 etc

HimiK
()

Even when we consider the repetition of numbers, each digit itself stands the chance to be any number i.e. 0 to 9

Sherlock
()

there are 7 digits. Each digit can have a unique value ranging from 0-9 (i.e. total ten different possible values).

Akshay
()

The questions suggests that the first four digits is dialed correctly, so neglect them. The last three digits is selected randomly from possible numbers 0 to 10. So, each position in the last three digits can take any number from 0 to 10 and also can be repetitive. That said the total number of possibilities for the last three positions is 10*10*10=1000.

So totally there are 1000 possible numbers and out of which only one number is correct. Therefore, P(correct number)=1/1000

Akshay
()

Please correct in the previous post the possible numbers are from 0 to 9 not 10.

Satyam
()

in question it is written only 7 digit no..how did u get 10???

HimiK
()

Lets only talk about the last three digits which can be any number i.e. 0 to 9. Hence possible numbers are 10.

Parul
()

from where did 10 came???