If m denotes the number of 5 digit numbers if each successive digits are in their descending order of magnitude and n is the corresponding figure, when the digits are in their ascending order of magnitude, then (m-n) has the value:

(A) ¹⁰C₄

(B) ⁹C₅

(C) ¹⁰C₃

(D) ⁹C₃

Hi,

The answer is 9C4.
For descending order : to form a five digit no we need to choose 5 nos out of 10 nos, which is 10C5. Now u dont need to arrange them, there is only one way by which 5 different nos can be arranged in only one way.
But for ascending order we have only 9 options to choose from, as including 0 and arranging it in ascending order will put on the front and with zero on front the no will be a 4 digit no and not 5 (For ex : if you choose 0, 1, 2, 3, 4 the ascending order will be 01234 but this is a four digit no and not 5) So for this case we have 9C4 cases.

So m-n gives, 10C5-9C5 = 9C4.