Adding decimals mentally, part 1 (two decimal digits; grade 4)
In this first part we add decimals that have two decimal digits, with mental math. This is easy as long as we think in HUNDREDTHS (think of them as fractions).
The 2nd part of the lesson deals with additions involving both tenths and hundredths, such as 0.2 + 0.68 (one number has 1 decimal digit; the other has two).