RetroJay, if in doubt just put any other value as the total weight, and you'll see that only 200 can be the answer.

You had 3 choices to pick from: 100, 150 and 200. And you now know the right answer is 200. Let's do the math for the other two, and you can do it for any other value if you still in doubt.

Let's say you choose 100. Half his weight would mean 100/2 = 50. Only 100+50=150... he can't have a total weight of 100.

Let's try 150 now... Half of his weight would be 75. 100+75 = 175. So his total weight can't be 150 either.

If you pick any other value, you will notice that you never get to a true value, except with 100 and total weight of 200.

But if it helps, I probably would have fallen for the trickery also and pick wrong. The riddle is phrased like that exactly to lead you into error thinking. Cause the first part makes your brain automatically think "he weights 100. And then add 50, that gives 150..." And even though this makes no sense (if he weights 100, he couldn't possibly also weight 150... but our brain accepts the first "fact" as true and thus gets stuck into figuring out the obvious.)