Answer:
 25%
Explanation:
The first thing is to identify the digits of 400, being 0 the first, the other 0 the second and the last digit would be 4, therefore, to make it less than 400, we have the following options: 
1 choice for 3rd digit, can only value 3, because 4 and 5 would be greater than 400. 
For the first and second digits it would be the same, two options, 4 or 5, no matter what. That is to say: 
2 choices (4,5) for 1st digit 
2 choices (4,5) for 2nd digit 
Therefore, so 4 possible options out of which 1 is correct 
P = 1/4 = 0.25 
P = 25%