Permutation Combination Question 8

Question 8 - 29 January - Shift 1

Five digit numbers are formed using the digits 1,2, 3, 5, 7 with repetitions and are written in descending order with serial numbers. For example, the number 77777 has serial number 1 . Then the serial number of 35337 is __________

Show Answer

Answer: 1436

Solution:

Formula: Permutation with repitition

No of 5 digit numbers starting with digit 1

$=5 \times 5 \times 5 \times 5=625$

No of 5 digit numbers starting with digit 2

$=5 \times 5 \times 5 \times 5=625$

No of 5 digit numbers starting with 31

$=5 \times 5 \times 5=125$

No of 5 digit numbers starting with 32

$=5 \times 5 \times 5=125$

No of 5 digit numbers starting with 33

$=5 \times 5 \times 5=125$

No of 5 digit numbers starting with 351

$=5 \times 5=25$

No of 5 digit numbers starting with 352

$=5 \times 5=25$

No of 5 digit numbers starting with 3531

$=5$

No of 5 digit numbers starting with 3532

$=5$

Before 35337 will be 4 numbers,

So rank of 35337 will be 1690

So, in descending order serial number will be

$3125-1690+1=1436$