you start at 20 and count back until you reach 1 (or zero). Each number is one less than the last. You can use number lines like the one below to help you learn how to count on and count back.

To make progress on one of number theory’s most elementary questions, two mathematicians turned to an unlikely source.

Counting On is a spinoff of 19 Kids and Counting, which originally featured Jim Bob and Michelle Duggar and their 19 kids: Joshua, twins Jana and John-David, Jill, Jessa, Jinger, Joseph, Josiah ...