I was composing the Karnaugh table for the expression $x'y'z'+x'y'z+x'yz'+x'yz+xy'z+xyz$.
The book has the answer:
My question is why the last row $1$'s are grouped as a double group whereas taking together above $1$'s could make a quarter and this is what we want. To make the groups as large as possible. Also groups can overlap. So what is the trick here?
First, whatever 'the answer' is when it comes to using Karnaugh maps totally depends on whatever goal you have. If the goal is simply to find some covering, then the book's answer is just fine.
But yes, typically we want to make our groups as large as possible. And as such, yes, you are right, you could have grouped that pair of $1$'s with the two above to make a group of 4, resulting in the expression $x'+z$