### Step 4: Check the units
Are you calculating:
* Years?
* Months?
* Days?
* Age?
* A calendar year?
A surprising number of mistakes happen because people calculate the correct number but attach it to the wrong quantity.
### Step 5: Read the result back into the sentence
This is the final test.
Put your answer back into the original question.
Does it make grammatical and logical sense?
If it doesn't, revisit your calculation.
## Sometimes the Trick Isn't Mathematical
That's another reason these puzzles are so effective.
The challenge may not require complicated arithmetic.
It may simply require recognizing that the question has been interpreted incorrectly.
For example, suppose a puzzle asks:
“Someone was 20 years old in 1990. How old were they in 2020?”
The answer is obviously:
[
2020-1990=30
]
The person's age in 1990 is not the number of years that passed.
The elapsed time is 30 years.
Their age in 2020 would be 50.
Now you can see how a puzzle could encourage someone to write “50 years” when the question actually asks for the **number of years between two dates**.
