Question

How many different four-digit numbers can be written using the digits $1,2,3,4,5,6,7$ and $8$. so that each of them contains only one unity, if any other digit can occur several times in the notation of these numbers?

Answer 1

In this case $2$ no. appears twice. As we have no's.

choose two places too the smaller no. that appear twice$⟹C_2^8\times C_2^6\times C_2^4\times C_2^2=2020$ ways$.$