LogFAQs > #958033681

It's pretty trivial, but can be hard to follow without visual aid (like colors or lines.) Basically find pairs that are mutually exclusive; sometimes if you keep looking, they form a complex path, like the example.

In this example, let's start with 4 in B1. There is exactly one other 4 in the same row. That means they are mutually exclusive; exactly one of them must be the answer. So assign them different groups or colors. Same thing if we look in the same column and same box.

Here the original 4 is in the green group, and all other 4's that are directly linked and mutually exclusive to it are in the yellow group. We continue this path with all mutually exclusive 4s that are linked to both of these groups. Every linked 4 will alternate between yellow or green.

Acknowledge that either the yellow or the green group is the correct answer. Any inconsistency for either group means that group is the wrong answer. Here the error is that two yellows will be in the same row. Therefore, yellow cannot be the correct answer. We choose the other group.
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