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First, we'll solve the left-hand side of the exercise since adding these numbers together gives us a round number:
This leaves us with a much easier exercise to solve:
120
\( 100-(30-21)= \)
Strategic grouping makes calculations easier! Since 13 + 7 = 20 (a round number), adding 100 becomes much simpler. This technique helps you work faster and make fewer errors.
That's great! Addition gives the same result regardless of order due to the commutative property. However, learning to spot easier combinations will help you solve more complex problems quickly.
Look for pairs that create round numbers (like 10, 20, 50, 100). In this case, 13 + 7 = 20 is much easier to work with than partial sums like 13 + 100 = 113.
No! Unlike addition, subtraction is not commutative. You must follow the order given in subtraction problems. This grouping strategy only works for addition.
Then simply add from left to right! The grouping strategy is just a helpful shortcut when it's available. Don't force it if the numbers don't naturally pair well.
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