If I use my scientific calculator I get 105 as a result

Markus

The thing about calculators is, you really need to read the manual for that specific calculator as there is no agreement between the various manufacturers how exactly their models interpret the input.

On some calculators, the above example may need paranthesis for the calculator to get it right, whereas on other models you won't need paranthesis.

For the record, it really is 105, which equals 7!!

And just in case there is any doubt:
7^2 - 3*7 = 7!!!

The key is that mathematics has its own language to include syntax.


In order to mislead the reader, the two languages (mathematics and English) were combined.

Which is equal to 7!!

Its less a mathematical ridlde and more a written communicational trap. It does not even work verbally, only in writing. If you shout (due to the two exclamation marks) "Seven!!", its still "seven". If you write "7!!", its "double factorial of seven".

Platapus wrote, as an answer, "7!!" Unsuspicous readers take the two exclamation marks as what they are in written language, exclamation marks. But here, they are expresison of a mathematical term, double factorial. 7!! means 1x3x5x7. And if you solve that, the result is 105.

The notation "7!!" refers to a double factorial. It is an extension of the regular factorial and is used in mathematics to represent a specific form of product calculations. Specifically:
The double factorial of 7 (i.e., 7!!) means:

7!!=1×3×5×7

This means multiplying all odd numbers up to 7. So:

7!!=1×3×5×7=105

For even numbers, the double factorial would look different. For example:

8!!=2×4×6×8=384

The double factorial refers to the product of all odd or even numbers up to the specified number.