Whole numbers are the set of positive integers. They do not have any decimal or fractional part.
Natural numbers along with zero(0) are whole numbers.
Addition of whole numbers always gives a whole number. Thus whole numbers are closed under addition.
Eg. 5+7 = 12
Subtraction of whole numbers does not always give a whole number. Thus whole numbers are not closed under subtraction.
Eg. 5-7 = -2, which is a negative integer and not a whole number.
Multiplication of whole numbers always gives a whole number. Thus whole numbers are closed under multiplication.
Eg. 5x7 = 35
Dividing a whole number by another does not always give a whole number. Thus whole numbers are not closed under division.
Eg. 5÷7 = a fraction, thus it will not be a whole number.