Question

How many $4-$digit codes can be formed by using the digits $1,2,3,4,5,6,7$ without repetition?

Answer 1

$\overline{7}\overline{6}\overline{5}\overline{4}$

no. of ways of filling $1^{st}$ place $=7$

no. of ways of filling $2^{nd}$ place $=6$

no. of ways of filling $3^{rd}$ place $=5$

no. of ways of filling $4^{rth}$ place $=4$

$\therefore$ By fundamental principal of

multiplication

$7\times 6\times 5\times 4=840$