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?

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Solution

The correct option is B $13$
Let $A$ be the set of children that have forgetten their rulers.

Let $B$ be the set of children that have forgetten their pencils.

$n (A \cup B) = 24$
$n (A) = 20$
$n (B) = 17$
$n (A \cup B) = n (A) + n (B) - n (A \cap B)$
$= 20 + 17 - 24$
$= 13$

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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?

The correct option is B 13Let A be the set of children that have forgetten their rulers.Let B be the set of children that have forgetten their pencils.n(A∪B)=24n(A)=20n(B)=17n(A∪B)=n(A)+n(B)−n(A∩B) =20+17−24 =13

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?

The correct option is B $13$
Let $A$ be the set of children that have forgetten their rulers.

Let $B$ be the set of children that have forgetten their pencils.

$n (A \cup B) = 24$
$n (A) = 20$
$n (B) = 17$
$n (A \cup B) = n (A) + n (B) - n (A \cap B)$
$= 20 + 17 - 24$
$= 13$