Here is a number sense trick to help you calculate addition using mental math.
We want to find the sum of 9 + 8 + 7. Using our number sense, we turn to “friendly numbers” such as 10 or multiples of 10, to help make this math easier to work with.
Here, all three numbers are close to 10 but slightly less than 10. We can make all three numbers ten by making them BIGGER and then subtract the extra amount we added to each number.
So, we make nine bigger by 1 to make ten, eight bigger by 2 to make ten, and seven bigger by 3 to make ten.
10, three times is 30, but we added six too much, so 30 minus 6 gives us a final answer of 24:
9 + 8 + 7 = 24.
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