One Common Pattern Leads to 45

A particularly clean way to solve the puzzle is to look at multiplication.

Consider the first equation:

1 + 4 = 5

You can interpret the result as:

1 × (4 + 1) = 5

Now try the same rule with the second equation:

2 × (5 + 1) = 2 × 6 = 12

It works.

Now apply it to the third:

3 × (6 + 1) = 3 × 7 = 21

It works again.

So the hidden rule can be written as:

First number × (Second number + 1)

Now we can apply exactly the same pattern to the final line:

5 + 8 = ?

Following the rule:

5 × (8 + 1)

= 5 × 9

= 45

So under this pattern:

The answer is 45.

But There Is Another Way to See the Pattern

There is an even more interesting way to approach the puzzle.

Look at the answers themselves:

5 → 12 → 21

The differences between them are:

12 − 5 = 7

and

21 − 12 = 9

The numbers are increasing by progressively larger odd numbers.

If the pattern continues, the next difference could be:

11

That would give:

21 + 11 = 32

But this produces a completely different answer.

And that’s an important lesson about these viral math puzzles: