Three are 6 different flags. Find the number of signals which can be formed with the help of at least 3 flags.

Explanation

No. of flag required for one signal =3 or 4 or 5 or 6.
Now number of ways to fill 3 places from 6 flags `= 6 xx 5 xx 4 = 120`
Similarly,
No. of signals containing flags `= 6 xx 5 xx 4 xx 3 = 360`
No. of signals containing 5 flags `= 6 xx 5 xx 4 xx 3 xx 2 = 720`
No. of signals containing 6 flags `= 6 xx 5 xx 4 xx 3 xx 2 xx 1 = 720`
Total flags `= 120 _ 360 +720 +720 = 1920`.