Notice what ... did with your return-calculation statement:
The /= stuff is shorthand...
This is the same thing to the compiler, and easier for humans to understand!
number = number / 2;
In C++ (and in most other programming languages) the following will add 10 to X:
X = X + 10;
Note that this is NOT an equation... it makes no sense in math. It is an assignment operation. It means "Assign a new value to X, which is 10 more than the current value."
The shorthand for this in C++ is:
X += 10;
In fact, you will see the += shorthand used in almost all "real" programs. But, to the compiler, they are EXACTLY the same.