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Trading cards — Ravi gives Zack 90
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?
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The pattern behind it: Constant total
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.
How to spot it
One person gives, sells or loses an amount to the other, and nobody outside the two people gains or loses anything.
Common mistake
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.
Try one like it
Standard Marbles — equal after a transfer Ali had 3 times as many marbles as Ben. After Ali gave 24 marbles to Ben, the two boys had the same number of marbles. How many ma…The 33 problem patterns
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
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