Question P4–P5–P6 Constant total
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Ravi had 7 times as many trading cards as Zack. After Ravi gave 90 trading cards to Zack, the two boys had the same number of trading cards. How many trading cards did Ravi have at first?
Answer
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Money moves between two people, so their total never changes.
In this one. A transfer keeps the combined total unchanged, so sharing the original 8 units equally makes 4 units each, and Ravi's 3-unit drop is exactly the 90 cards transferred.
One person gives, sells or loses an amount to the other, and nobody outside the two people gains or loses anything.
Children draw different totals before and after, or forget that moving an amount from one person to the other changes the gap between them by twice that amount.
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