[murderer mentality]
To solve a complex word problem, you must oftentimes work backwards.
Look up the answer in your textbook. Understand the mathematics behind it by following it from creation to visualization. In other words, if you get lost in the forest, retrace your steps via the bread crumbs already laid out for you.
L knows this much better than anyone. Logic has been his forte, math and science merely subcategories of his vast mental processing capability. He sees the hidden numbers, understands what appears to be meaningless arrays of jargon. Because, in his time as detective he has discovered that nothing is ever done without a reason.
There is a reason the good serial killers leave clues, after all. It is to taunt the police, to establish dominance. It is to taunt him, and only the true idiots will allow themselves to be goaded and led around like lambs by a wolf. Therefore, it is vital that he not only analyze the answer, but the steps taken to achieve the end result. Why did this killer choose his targets? What is the meaning of the single apple left behind by the victim? Where is the killer hiding? Behind the numbers lies the truth, and the truth is often closer than you think.
The world is rendered not in black and white, but in shades of grey, after all.
To understand a killer, you must think like a killer.
He has sunk to the depths of depravity, pondered the rational methodology each killer is imbued with. He analyzes the choice of victims and makes connections that only the killer himself would think of, would conceive.
To think like a killer, you must deny the existence of the outside world and enter the lowest levels of solipsism; to perceive reality as a dream, where only your mind is real.
To think like a killer, you must become a killer.
There is a reason why Beyond Birthday had such stark similarities with him, after all.