Problem Solving Skills for Mathcounts Competitions

Book Store => Bookstore => Twenty Mock Mathcounts Sprint Round Practices => Topic started by: mathcount on February 15, 2011, 10:04:27 AM

Title: Mock Mathcount
Post by: mathcount on February 15, 2011, 10:04:27 AM

How many three digit positive integers are there such that the three digits are in increasing order (like 145) or Decreasing (like 321)?

if we look at it 123 (7!)
                     234 (6!) because 235,236 to 9 and then 3 goes up to 6

But the asnwer is 204

Please help

Title: Re: Mock Mathcount
Post by: yongcheng3315 on February 15, 2011, 11:15:54 AM

Therer are 84 increasing numbers and 120 decreasing number.

See Twenty More Problem Solving Skills For Mathcounts Competitions

Chapter 5 Some Special Numbers for a short-cut.

Title: Re: Mock Mathcount
Post by: mathcount on February 15, 2011, 11:53:10 AM

Thank you
 I got the book