Try it against all the examples.

Step 4: Keep the rule simple

When several patterns are possible, the simplest consistent rule is often the intended one.

Step 5: Remember that some puzzles are ambiguous

If two different rules fit equally well, there may not be a uniquely provable answer.

So, What’s the Answer?

Using the most straightforward multiplication-based pattern:

1 × (4 + 1) = 5

2 × (5 + 1) = 12

3 × (6 + 1) = 21

Therefore:

5 × (8 + 1) = 45

Answer: 45

But there’s an important caveat.

The puzzle does not mathematically establish that 45 is the only possible answer. Another consistent pattern can produce 34, for example, by incorporating the previous result.

So if this is a viral brain teaser and you’re asked for the intended answer, 45 is a strong choice.

The bigger lesson is perhaps more interesting than the number itself: sometimes solving a puzzle isn’t about calculating faster. It’s about being willing to question the assumption that the symbols mean exactly what they appear to mean.

The next time you see a simple-looking equation, don’t just calculate it. Look for the rule hiding underneath.