Final answer:
To get back his original number, Jonas needs to divide the product by 5. Dividing the product by 5 will undo the multiplication operation.
Step-by-step explanation:
To get back his original number, Jonas needs to divide the product by 5. Since he multiplied his number by 5, dividing it by 5 will undo that operation and return him to his original number.
For example, if Jonas thought of the number 3 and multiplied it by 5, the product would be 15. To get back to the original number 3, Jonas should divide 15 by 5, which equals 3.