Number 16. A triangular pyramid is built with 10 identical balls as shown. Each ball has one of the letters A, B, C, D, and E on it. There are two balls marked with each letter. The picture shows three side views of the pyramid. What is the letter on the ball with the question mark? Okay, so each side has a matching edge with the other side views. Like, as you can see in this picture, if we have a side view over here, these three are also going to be the same on this side view over here. And also, this will be the same as the side view that we can't see right now from this face over there. And this side view from this one will be the same as the back as well. So, that's the first thing we know. The second thing we know is that we have 10 identical balls, and each ball with one of these letters, so there should be two of each ball. So, what we can do is we can cross out the duplicate edges. Here we see that we have A, B, D, so we can cross out this A, B, D over here. Then we see we have an A, C, E, so we can cross out this A, C, E over here. We have our A, E, D over here, so we can cross out the A, E, D over here. And now we only have one copy of each one. So, this is all 10 balls that we have. Now we know that there are two of each letter, so we can count. We see that there are two B's, there are two C's, there are two D's, there are two E's, but there's only one A. So, this one on the bottom must be an A.

triangular pyramid

missing letter

letter distribution

duplicate elimination

ball puzzle