Explanation

(b)
Number greater than 1000 and less than or equal to 4000 will be of 4 digits and will have either 1 (expect 1000) or 2 or 3 in the first place with 0 in each of remaining places.
After fixing 1st place, the second place can be filled by any of the 5 numbers.
Similarly, third place can be filled up in 5 ways and 4th place can be filled up in 5 ways. Thus, there will be $25 \times 5 = 125$
Ways in which 1 will be in first place but this include 1000 also hence there will be 124 numbers having 1 in the first place. Similarly, 125 ways for each 2 or 3 in the Ist place.
One number will be in which 4 in the first place $ie$, 4000
Hence, the required number of numbers
$= 124 + 125 + 125 + 1 = 375$