Start from zero — the basic math you need before everything else
A decimal is another way to write a fraction whose bottom is 10, 100, 1000, and so on. The digits after the dot have their own place values: the first is tenths, the next hundredths, then thousandths. So 0.5 is five tenths, which is the same as 1/2.
To add or subtract decimals, line up the dots so tenths sit under tenths and ones under ones, then work as usual. To turn a fraction into a decimal, divide top by bottom (or rewrite it over 10 or 100). To go back, read the decimal as a fraction and simplify.
Decimals are everywhere you measure. A ruler shows 1.5 cm as one whole centimetre and five of the ten little marks (five tenths). Kitchen scales read 0.25 kg as a quarter of a kilo. A stopwatch shows 9.4 seconds — nine whole seconds and four tenths.
▶ Decimals