Question

How many six-digit numbers contain exactly four different digits?

Answer 1

Let there be $10$ digits i.e, $0,1,2,3,4,5,6,7,8,9.$

Let $AAABCD$ and $AABBCD$ be exactly $4$ different digit

$\Rightarrow \{(C_1^{10}\times 1\times 1)\times C_3^9\times \frac{L^6}{L^3}+C_2^{10}\times C_2^8\times \frac{16}{L^2\cdot L^2}\}\times 0.9=294840$