Thread: [SOLVED] combinatorics

1. How many 3-digit positive integers are odd and do not contain the digit 5?
2.Martha invited 4 friends to go with her to the movies. There are 120 different ways in which they can sit together in a row. In how many of those ways is Martha sitting in the middle?

Originally Posted by william

1. How many 3-digit positive integers are odd and do not contain the digit 5?
2.Martha invited 4 friends to go with her to the movies. There are 120 different ways in which they can sit together in a row. In how many of those ways is Martha sitting in the middle?

1.4*9*8=288 ways
2.24*1=24 ways

...Can you explain why?