Final answer:
The bi-conditional statement for the given conditional is: A number is a natural number if and only if it is a whole number.
Step-by-step explanation:
The conditional statement 'if a number is a natural number then it is also a whole number' can be expressed as an if-then statement: If a number is a natural number, then it is a whole number. To obtain the bi-conditional statement, we need to reverse the direction of the conditional and combine it with the original conditional. The bi-conditional statement would be: A number is a natural number if and only if it is a whole number.