If there are 10 girls, the handshakes go as :

1st girl = 9

2nd girl = 8

.....

10th girl = 0

So, it is 9+8+.....+1+0 = Sum of natural numbers upto 9 = 9*10/2 = 45

Now, if we use the formula of sum of natural numbers taking N=10,

then, N (N+1)/2 , for 10 girls becomes 10 * 11 / 2 = 55.

Whereas, the question asked is about handshakes between the girls and not the girls,

we need to consider 9 + 8 +....... + 1

which boils down to N (=9 ) * N+1 (=10) / 2

But, if we want a direct answer, we can take N as 10 girls, in which case

formula has to be considered as N(N-1)/2.

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SAILAJA KATTUBADI