Adding and taking away tenths

Sonu measures his arm. Lower arm: 2 and 7/10 units. Upper arm: 3 and 6/10 units. Total?

Way 1: add wholes to wholes and tenths to tenths. (2 + 3) = 5 wholes, (7 + 6) = 13 tenths. But 13 tenths is more than a whole: it is 1 whole and 3 tenths. So the total is 6 and 3/10 units.

Way 2: change both to tenths first. 2 and 7/10 = 27/10; 3 and 6/10 = 36/10. Add the tops: 63/10 = 6 and 3/10. Same answer.

Careful: add only the tops. The bottom stays 10. (Not 10 + 10 = 20!)

Try a honeybee: head 2 and 3/10, thorax 5 and 4/10, abdomen 7 and 5/10 units. What is its whole length?

Now subtraction. Shylaja's hand is 12 and 4/10 units and her palm 6 and 7/10. How long is her middle finger? We need (12 and 4/10) − (6 and 7/10). We can't take 7 tenths from 4 tenths, so borrow one whole as ten tenths: 12 and 4/10 = 11 and 14/10. Now: 11 − 6 = 5, and 14 − 7 = 7 tenths. The middle finger is 5 and 7/10 units.