Hopping Backward on a Number Line to Show Subtraction

You will meet this idea in ordinary days. A thermometer falling on a cold night is subtraction on a number line, and everyone can see it happening. This is worth learning because one picture now shows both operations, which keeps them straight in your mind.

Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look. Here is today’s math treasure. Subtraction on a number line goes the other way.

For 12 minus 5, put your finger on twelve and hop backward five times: eleven, ten, nine, eight, seven. You land on seven. Backward hops mean taking away. If you land left of zero, something went wrong, because you cannot take away more than you had in these problems. Count the hops carefully and check your landing.

Try both directions on one number line. Start at ten, hop forward three to thirteen, then hop back three and land on ten again. Seeing yourself return to the start shows you that addition and subtraction undo each other, which you will use constantly from here on. Take your time. Careful is better than fast.