Try this beautiful Problem based on combinatorics from AMC 8 2020.
How many integers between 2020 and 2400 have four distinct digits arranged in increasing order? (For example, 2347 is one integer.
Combinatorics
Counting
AMC 8 2020 Problem 7
15
The second digit can't be 1 or 2, since the digit need to be increasing and distinct , and the second digit can't be 4 also since the number need to be less than 2400, so its 3
now we need to choose the last two digit from the set 
now we can do it in 
ways. now in only one way we can order so there are 15 numbers.
Try this beautiful Problem based on combinatorics from AMC 8 2020.
How many integers between 2020 and 2400 have four distinct digits arranged in increasing order? (For example, 2347 is one integer.
Combinatorics
Counting
AMC 8 2020 Problem 7
15
The second digit can't be 1 or 2, since the digit need to be increasing and distinct , and the second digit can't be 4 also since the number need to be less than 2400, so its 3
now we need to choose the last two digit from the set
now we can do it in ways. now in only one way we can order so there are 15 numbers.
