lesson
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Imagine you have 7 delicious strawberries on a plate. You look away for a second, and suddenly only 4 are left!
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How can we figure out how many strawberries went missing?
Finding the Missing Middle Number
When the middle number is missing, like in 7−=4, we start with the whole group and take away what is left.
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Since 7−4=3, we know that 3 strawberries were eaten!
What if we don't know how many we started with at the very beginning?
Finding the Missing Start Number
Look at this mystery: −3=5. We lost 3, and we still have 5 left.
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To find the total start, put both parts back together by adding: 5+3=8.
Be careful: never subtract 5−3 here, because the starting number must be bigger than the parts you took away!