Question

How many different four-digit numbers can be written using each of the digits $1,2,3,4,5,6,7$ and $8$ only once so that each number should contain a unity?

Answer 1

First place the unity in the four positions wich can be done by $4$ ways then then fill the 3 place using remaining $7$ numbers which can be done in $7\times 6\times 5=210$ ways

$\therefore$ Total number of ways are $840$