考考你: $10哪去了? Where is $10?

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John borrows $500 from dad and $500 from mom. He uses $970 to buy a pair of shoes, balance $30.
He returns $10 to dad, $10 to mom, keeps $10 for himself.
So he owes $490 to dad, $490 to mom,
$490 + $490 = $980,
with his $10 = $990.

Where is the missing $10 (= $500 + $500 – $990) ?

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