Adding and comparing decimals
Line up the decimal points and pad the shorter number with zeros so every column holds the same place value.
A worked example
For 6.4 + 1.27, write 6.40 above 1.27 so the hundredths line up, then add to get 7.67.
The mistake people actually make
More digits does not mean a bigger number. Compare place by place from the left: 0.9 is larger than 0.85.