<p>I guess and checked, but I made educated guesses. I figured that C = 1, because there aren't two three-digit numbers whose sum would equal a number in the 2000s (999 + 999 = 1998). </p>

<p>So if C = 1, then I could make a guess on A and B. I started with A = 9 and B = 2, because 9 + 2 = 11 and this would give me a 1 in the ones digit:</p>

<p>999</p>

<h2>+222</h2>

<p>1221</p>

<p>This would work, but B and D are the same, and they must be different. I tried A = 8 B = 3, again because 8 + 3 = 11, giving me a 1 in the ones digit:</p>

<p>888</p>

<h2>+333</h2>

<p>1221</p>

<p>Now all the numbers are different and D = 2</p>