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The student adds round numbers in the head by adding the short part and restoring the zeros.
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The three steps from page 23 for adding round numbers.
Mental math can save you a lot of time in everyday calculations. Let's look at adding using a quick mental shortcut.
Step 1: Check that the addends are in the same grouping. Here, both are tens.
The Golden Rule: This trick only works as a reliable shortcut when every addend has the same number of zeros on the right.
What to do when the small addition goes past nine.
What happens when the short parts add up to 10 or more? You follow the exact same steps!
Let's look at . Both numbers are grouped in hundreds (two zeros). We simply add the short parts:
Then, just put the two zeros back on the end of the whole thing to get .
The two further examples from page 23, worked in the same three steps.
Problem. Add the numbers: .
Step 1. Check the groupings. All addends are in hundreds, meaning they all share exactly two zeros on the right.
Step 2. Isolate and add the short parts.
Five items from Worksheet 3, covering every grouping.
Let's practice the mental addition steps with different groupings. When adding , the numbers are in tens, so the short sum is , giving a final answer of . For , the numbers are grouped in hundreds, yielding a short sum of and a total of . If we add , the short parts add to , giving us . Moving to thousands, gives a short sum of and a total of . Finally, adding creates a short sum that ends in zero, resulting in as the total.