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n' similar balls each of weight w, are weighted in pairs. The sum of the weight of all the possible pairs is 120. When weighted in triplets the sum of the weights is 480. Then n divides:

A

`25`

B

`30`

C

`60`

D

`75`

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The correct Answer is:
B, C
Total number of selection in paris
`(n(n-1))/(2)and hence`
`120=(n(n-1))/(2).2w.`
Total number of selection in triplets
`=(n(n-1)(n-2))/(6)` and hence
`480=(n(n-1)(n-2))/(6).3wimpliesn=10`
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