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  4. In a mathematics class, 20 children had forgotten their rulers and 17 had forgotten their pencils, Go and borrow them from someone at once, said the teacher, 24 children left the room, then how many children had forgotten both is

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Q. In a mathematics class, 20 children had forgotten their rulers and 17 had forgotten their pencils, "Go and borrow them from someone at once", said the teacher, 24 children left the room, then how many children had forgotten both is

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Solution:

$n(R) = 20 , n(P) = 17$
$n(R \cup P) = 24$
$n(R \cap P) = 20 + 17 - 24 = 13$