Adding decimals mentally (two decimal digits) - 4th grade math
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).
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In this 2nd part, we learn how to add decimals (with mental math) when one number has 1 decimal digit and the other has 2, such as 0.3 + 0.55. Some children add 3 + 55 = 58, and give the result as 0.58. Instead, we need to think of 0.3 or three tenths as 0.30 (30 hundredths). Then the addition is easy: 0.30 + 0.55 = 0.85.
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See also
Decimals with hundredths (two decimal digits) plus comparing them (2 videos)
Math Mammoth Grade 4 curriculum