"Find how many numbers greater than 2500 can be formed from the digits 0,1,2,3,4 if no digit can be used more than once.
Please explain in details.
I find it so hard to understand this topic."
Clearly the only way you can get a larger number than 2500 is to either start with the 3 or the 4, any other starting number would be too small
now consider that you start with the number 3, it doesnt mater what order the other numbers go in the final number will always be bigger than 2500.
so you need to find the number of combinations that you can arrange 0,1,2,4 which is given by n!
so in this case is 4!=24
it is exactly the same if you start with 4
so your final answer will be
hope this helps