Divide decimals with mental math, part 1: sharing divisions (whole-number divisor)
A lesson on sharing divisions with decimals that have up to 3 decimal digits (and naturally a whole-number divisor). I first use a visual (concrete) model, but actually we can solve many of these by using simple mental math. For example, 0.24 ÷ 6 can be thought of as 24 hundredths shared equally between 6 people — each person will get 4 hundredths, so the answer is 0.04.
Sometimes, it helps to add a decimal zero to the dividend. For example, to solve 0.4 ÷ 5, write 0.2 as 0.40 first to get 0.40 ÷ 5. Now, it is 40 hundredths divided by 5 to get 8 hundredths, or 0.08.
These are very simple principles, yet often forgotten in the typical school curriculum. I intend this lesson for 5th grade and onward — whenever students have gained a good basic grasp of decimal place value. It also matches the Common Core Math Standard 5.NBT.7
The second part of this lesson deals with mental math divisions where the divisor itself is a decimal number.
See also
Math Mammoth Decimals 2 — downloadable worktext
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