Permutations Practice Test 1

Question : 31 of 40

Marks: +1, -0

ORIGINAL QUESTIONS FROM PREVIOUS YEAR PAPERS

There is a seven-digit telephone number withall different digits. If the digit at the extremeright and extreme left are 5 and 6 respectively,find how many such telephone numbers arepossible?

120
1,00,000
6,720
30,240

Validate

Solution:

There is a seven digit telephone number and 5 and 6 are on extreme right and extreme left.
So, the number 0, 1, 2, 3, 4, 7, 8, 9 will lie between these numbers.
So,

^8C_5=8!/{5!3!}=56

We can represent the five numbers in 5! ways = 120
So, numbers of possible telephone number = 120 × 56 = 6,720

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