Since the first digit is 5, the other three digits must sum to 5

If the second digit is 0 the other two digits must sum to 5, so they
are 0+5, 1+4, or 2+3 or their reverses 5+0, 4+1, amd 3+2, that 6

If the second digit is 1, the last two digits must sum to 4, so they
are 0+4, 1+3, or 2+2.  The first two have reverses, so that's 5 more

If the second digit is 2, the last two digits must sum to 3, so they
are 0+3 and 1+2, and their reverses, so that's 4 more.

If the second digit is 3, the last two digits must sum to 2, so they
are 0+2 and 1+1 and the reverse of the first, so that's 3 more.

If the second digit is 4, the last two digits must sum to 1, so they
are 0+1 and its reverse, so that's 2 more

If the second digit is 5, the last two digits must sum to 0, so they
are 0+0, so that's 1 more.

Total = 6+5+4+3+2+1 = 21.


 1.  5005
 2.  5014
 3.  5023
 4.  5032
 5.  5041
 6.  5050
 7.  5104
 8.  5113
 9.  5122
10.  5131
11.  5140
12.  5203
13.  5212
14.  5221
15.  5230
16.  5302
17.  5311
18.  5320
19.  5401
20.  5410
21.  5500

Edwin