Adding quantities with tenths

The idea

To add quantities given in units and tenths, add the units and the tenths separately and regroup every 10 tenths as 1 unit; alternatively, convert both quantities to tenths, add them, and regroup.

Sonu’s arm

Sonu’s lower arm is 2 7/10 units long and his upper arm is 3 6/10 units long.

What is the length of his whole arm?

• Between 5 and 6 units
• Between 6 and 7 units
• Exactly 5 units

Add, then regroup

Add place by place. When the tenths column has 10 or more, tap 10 → 1 to make a unit.

Two ways to add

Both ways give the same total for the arm.

Units and tenths separately: (2 + 3) + (7/10 + 6/10) = 5 + 13/10 = 5 + 1 + 3/10 = 6 3/10

All in tenths: 27/10 + 36/10 = 63/10 = 6 3/10

To add quantities in units and tenths, add the units and the tenths separately, and regroup every 10 tenths as 1 unit. Or change both quantities into tenths, add, and regroup.

Notes

To add units and tenths: add the units and the tenths separately, then regroup 10 tenths into 1 unit. Or change both to tenths, add, and regroup.

Check yourself

What is 1 4/10 + 2 9/10? Type a mixed number, like 2 1/10.

Answer: 4 3/10

3 units and 13 tenths = 3 + 1 unit and 3 tenths = 4 3/10.

Which is not a correct way to add 2 7/10 and 3 6/10?

A rope is 4 5/10 m and another is 3 8/10 m. Change both to tenths and add. How many tenths in all?

Answer: 83 tenths

45 tenths + 38 tenths = 83 tenths = 8 units and 3 tenths = 8 3/10 m.

• Add the units (5) and the tenths (13), then regroup 13 tenths. This is correct: 5 + 13/10 = 6 3/10.
• Change both to tenths: 27 tenths + 36 tenths = 63 tenths. This is correct: 63 tenths = 6 units and 3 tenths.
• Add 27 + 36 = 63 and call the answer 63 units — correct. Yes, this is wrong. 27 and 36 are counted in tenths, so 63 means 63 tenths, not 63 units.