A sequence can often be made to fit more than one mathematical rule.

Without additional information specifying the intended rule, there may not be a single mathematically unique answer.

Another Popular Solution Uses the Previous Result

There is another pattern that many people use when solving puzzles like this.

Start with:

1 + 4 = 5

Then take the previous answer and incorporate it into the next equation:

2 + 5 + 5 = 12

Then:

3 + 6 + 12 = 21

Following that same idea:

5 + 8 + 21 = 34

This method gives:

34

So now we have two plausible answers:

45 using one rule
34 using another rule

Which one is correct?

The problem is not necessarily your mathematical ability.

The real challenge is that the puzzle doesn’t explicitly tell you what kind of pattern you’re supposed to find.

In mathematics, a finite set of examples can often be explained by many different rules.

For example, if someone gives you three points on a graph, there can be many mathematical functions passing through those points.