If Casey does not buy a medium yellow shirt, which one of the following must be true?

kmccarthy10 on July 15, 2020

Can someone go through why this answer is correct?

I know someone asked this already, but it hasn't been answered

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shunhe on July 15, 2020

Hi @kmccarthy10,

Thanks for the question! So we’re told here that Casey doesn’t buy a medium yellow shirt, and then asked what must be true.

Theoretically, we have nine total shirt combinations.
S = small, M = Medium, L = Large
R = Red, Y = Yellow, B = Blue

SR MR LR
SY MY LY
SB MB LB

But we’re told in the game that there’s no SR or LB, so there’s really 7 combinations.

XX MR LR
SY MY LY
SB MB XX

Now notice something here. Remember that we can’t buy S if we buy L, and vice versa. Let’s say we buy an S. Then we can’t buy any of the L shirts. And we need to buy two more shirts. But there’s only one other S shirt available. This means we have to buy at least one M shirt.

Similarly, since it works in reverse, if we buy an L, we can’t buy any of the S shirts. Again, we need to buy two more shirts. But again, there’s only one other L shirt available, so we need to buy at least one M shirt. So no matter what, we have to buy at least one M shirt!

What does this mean? Well, let’s say we don’t buy MY. We know from above that we have to buy at least one M. So if it’s not MY, it has to be MR or MB! And that’s what (B) says, which is why (B) is the correct answer.

Hope this helps! Feel free to ask any other questions that you might have.

kmccarthy10 on July 15, 2020

thank you!

shunhe on July 17, 2020

No problem!