The problem is as follows:
The following board must be completed with natural numbers from $1$ to $15$ with no repetitions allowed in such a way that the sum of the numbers on each of the three squares of $3 \times 3$ is always the same. However, the sum of the numbers on each of the $3 \times 3$ squares must be the maximum as possible. What will be the result of that sum?
The options to choose from given are:
$\begin{array}{ll} 1.&87\\ 2.&86\\ 3.&89\\ 4.&88\\ \end{array}$
I've found this question in my riddles book. I've attempted to solve it but I've became tangled with different trials. Does there exist a strategy or straightforward method to solve this? Can somebody help me?
I believe what it is intended to be asked here is to attain the maximum sum in those areas which are bounded by the bold lines.
Each region is composed of nine numbers. Then I tried to select it from $1$ to $9$ as follows:
$1+2+3+4+5+6+7+8+9=45$
and the rest:
$10+11+12+13+14+15=75$
But I don't know if what I'm doing is right or not. Supposedly the answer is $86$, but I have no idea how to get there and more importantly why?.
I'd really appreciate somebody could include some drawing or sketch as part of your answer so I could better visualize how should I fill these squares or what sort of logic should be used to solve this.