Solution: Let's figure out the answer by eliminating the ones we know are wrong. Choice (A) can't be right because then both children would be wrong. Choice (B) can't be right because Juan would be wrong, and we can eliminate choice (C) because it would mean that Lashana were wrong. But if we have eliminated (A), (B), and (C), then all we have left is (D). How could it be right? Choice (D) could be right if there were 3 horses on one side and 4 on the other, so that MUST be true!
The problems and the techniques are from the book Avoid Hard Work! … And Other Encouraging Mathematical Problem-Solving Tips for the Young, the Very Young, and the Young at Heart by Maria Droujkova, James Tanton, and Yelena McManaman. For more information, see http://naturalmath.com/avoid-hard-work/.