First slide

Combinations

Question

A teacher takes 3 children from her class to the zoo at a time as often as she can, but she does not take the same three children to the zoo more than once. She finds that the number of times she goes to the zoo is 84 more than a particular child goes to the zoo. Number of children in her class is

Moderate

A

12
B

10
C

60
D

40

Solution

No of times teacher goes to the park $=^{n} c_{3}$
No of times a particular child goes to the park $=^{n - 1} c_{2}$

G.T $^{n} c_{3} = 84 +^{n - 1} c_{2}$

$\Rightarrow n = 10 (b y v e r)$

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