Question:
How many different numbers of six digits each can be formed from the digits 4, 5, 6, 7, 8, 9 when repetition of digits is not allowed?
Solution:
Number of ways of filling the first digit = 6
Number of ways of filling the second digit = 5 (as repetition is not allowed)
Number of ways of filling the third digit = 4
Number of ways of filling the fourth digit =3
Number of ways of filling the fifth digit = 2
Number of ways of filling the sixth digit = 1
Total numbers $=6 \times 5 \times 4 \times 3 \times 2 \times 1=720$
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