I don't remember the full question. But I am hoping someone has come across the same question. This is how it is worded.
There are 3 products, X, Y ans Z. A table shows these products and their corresponding production times. Product X and Z can be produced together but Y and Z cannot be produced at the same time. If so, how long will it take to produce 20 numbers of X, 15 numbers of Y and 30 numbers of Z?
It is a question related to maximizing or minimizing something. I am giving my opinion. Others may add to it.
If you want to minimize time, you need to see how you can reduce the task taking maximum time. So essentially you will try to execute X and Z or X and Y together (you cant execute Y and Z). Now take each answer and see if it can be done in that time frame.
Umm I think this should give you one way of approaching this problem.