Rounding to a whole number means rounding a decimal digit number to the nearest whole number and eliminating the decimal digit.
To do so we should have a look at the decimal digit
Here we have 13.2
here the 2 is the digit in the decimal
We already know that when the number to be rounded is less than 5 then it is rounded to the whole number preceding it and if it is greater than 5 it is rounded to the whole number next to it.
Here we can see that 2 is less than 5 , and it is closer to 13 than to 14. So we can round this number to 13 feet
( Extra point for better understanding: If we have say 13.8 to be rounded to nearest whole number, we can see that 8 is greater than 5 and so it is closer to 14 than to 13 then it will be rounded to 14)