Yvon has 4 different notebooks and 5 different pens. He must bring exactly one notebook and exactly one pen to his class. How many different possible combinations of notebooks and pens could he bring?
Problem 151
Pick one
Official solution
Since the third and fourth numbers in the list are consecutive and add to 11, then the third number in the list is 5 and the fourth is 6.
The fifth number in the list is 7, and so the sixth number in the list is 8.