Question

There are 21 students in a class.If every student studying in the class give a greeting card to every other atudent in the class,then how many greeting cards are exchanged

Answer 1

21 (students) x 21 (cards) = 441 cards are exchanged please mark brainliest

Answer 2

Answer:

Greeting cards are exchanged in 420 ways.

Step-by-step explanation:

• If 2 students exchange a greeting card, one student to send card and the other student to receive the card.
• 2 students can be selected out of 20 students in 20C₂ ways.

• And two students can exchange cards each other in two ways,

The total number of greeting cards exchanged by the students =

ways.

$C(n,r) = \binom{n}{r} = \frac{n!}{( r! (n - r)! )}$

$21C_2= \frac{21!}{( 2! (21 - 2)! )}$

         =210

$2\times 21C_2= 420$ WAYS.