Hey! :)
First, let's get to know about rounding.
Rounding, let's look at a example of rounding. We have a decimal 23.44 for example, it is not above 50 meaning it rounds to 23.00.
Now, let's look at our equation.
As we can see, 21.58 is above 50 so it rounds to the next number, our answer is 22.00.
Hopefully that's clear!
Have a good one :)