A4. How many positive integers less than 1000 are there such that the sum of the digits is equal to 6?
(A) 7
(B) 16
(C) 27
(D) 28
(E) 35
A4. $6=0+0+6=0+1+5=0+2+4=0+3+3=1+1+4=1+2+3=2+2+2$

With 0,0 and 6 we can make three numbers, 600, 60, and 6, where leading zeros have been omitted. Similarly, we can make three numbers with 0,3 and 3, and also with 1,1 and 4.

With 0,1 and 5 we can make six numbers; similarly with 0,2 and 4, and also with 1,2 and 3.

With 2, 2 and 2 we can only make the number 222.

In total, therefore, $3 \times 3 + 3 \times 6 + 1 = 28$, so the answer is (D)