This question stem is asking us to identify which pair of trucks must share a building in common.
The global setup one can have is this:
I found it prudent to show 2 scenarios. We have 1 Y left to distribute. It can go in one of I and S.
It is important to remember that in both of those frames above, there is no more Y's to give out. I also placed barriers next to variables if that truck is maxed out.
So in answering #19, I can see that P must serve a common building with I and T. The test writers placed the P and I commonality. This must be true.