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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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