Adding decimals, and where decimals go wrong

Priya needs 2.7 m of cloth for a skirt, Shylaja 3.5 m for a kurti. Together: 2.7 + 3.5 = 6.2 m. How much more does Shylaja need? 3.5 − 2.7 = 0.8 m. Add and subtract decimals just like whole numbers: line up the decimal points (tenths under tenths, ones under ones), and carry or borrow as usual.

It works for any size: 75.345 + 86.691 = 162.036. Try 84.691 − 77.345.

Sequences: 4.4, 4.8, 5.2, 5.6, 6.0, ... goes up by 0.4, so next come 6.4, 6.8, 7.2. Continue: 4.4, 4.45, 4.5, ... · 5, 4.95, 4.90, ... · 36.5, 33, 29.5, ...

Estimate first. Sonu says the sum of two decimals is more than the sum of their whole parts and less than 2 more. For 25.936 + 8.202: between 33 and 35. The answer, 34.138, fits. It always works, because each decimal part is less than 1.

A wrong decimal can be a disaster:

• 2013, Amsterdam: the city sent out €188 million in housing benefits instead of €1.8 million, because a program read cents as euros.
• 1983: an Air Canada plane was fuelled with 22,300 pounds of fuel instead of kilograms, about half what it needed. It ran out in the air and landed at an old airfield; everyone survived.
• Reading 0.05 mg as 0.5 mg gives ten times the medicine.

Some numbers only look like decimals. The bus arrives 4.5 hours past noon means 4:30, not 4:50 (half an hour is 30 minutes). A doorway of 2 ft 5 inches is not 2.5 ft (0.5 ft is 6 inches). In cricket, 5.5 overs means 5 overs and 5 balls, because an over has 6 balls, not 10.