Adding with Decimal Fractions

Using Materials

Problem: Work out 3.9 + 2.4

Students model 3.9 + 2.4 as 3 wrapped bars (wholes) and 9 small pieces (tenths) and as 2 wrapped bars and 4 small pieces. Collecting them gives 5 wholes and 13 tenths. By the canon of place value, 10 tenths are rewrapped to give 1 whole.

So 3.9 + 2.4 = 6.3

Using Materials, Using Imaging, and Using Number Properties

When teachers make up their own examples, they should generally make sure that there are 10 or more tenths in the addition so that 10 tenths has to be swapped for 1 whole. So 3.3 + 1.9 is suitable, but 3.4 + 4.4 is not. The teacher can select problems from the set below for any stage.

3.9 + 2.4      2.6 + 1.6         1.4 + 0.8        3.5 + 2.5

0.6 + 1.7      5.8 + 0.8         2.2+ 1.9         4.2 + 2.3

2.9 + 1.9      0.6 + 1.6         3.2 + 0.8        2.4 + 1.5

10.4 + 10.9         18.5 + 1.7            0.4 + 20.8                   15.2 + 2.3