Multiplying round numbers and calculating with decimals
Multiply numbers that end in zeros (40 × 30, 650 × 300), add and subtract decimals in columns, and multiply two decimals by counting decimal places.
- France: grade 4
- Germany: grade 3
- Iran: grade 5
- Palestine: grade 3
- Ukraine: grade 5
Guide
To multiply numbers like 40 × 30, set the end zeros aside and work out 4 × 3 = 12. Then write all the zeros of both numbers after the answer: 40 × 30 = 1200. In 800 × 200 there are two zeros and two zeros, so 8 × 2 = 16 and four zeros: 160 000.
Column multiplication does the same thing: to multiply by 300, write a 0 in the ones column and a 0 in the tens column, and then multiply by 3.
To add and subtract decimal (decimal number)s, line up the decimal point so tenths are under tenths and hundredths under hundredths. Fill any gap with a 0: 5.2 = 5.20. Start on the right. If the top digit is too small, exchange from the column on the left, even if that column is across the decimal point.
To multiply two decimals, forget the decimal points and multiply as whole numbers. Then count the decimal places in both numbers together and cut off that many places from the right.
Every digit you get right turns green. If you get stuck, press "Show me this column".