Data Sufficiency

gmat prep tough 2

exaam tommm

It should be 3/40,000

Total 60 pairs of siblings, so 60 of them are juniors and the other 60 of them are seniors.

So choosing all 60 junors out of 1000 juniors is =60/1000

and then choosing every senior out of 800 seniors is 1/800 (because the other set is having all juniors considered out of 1000)

you can choose either way.. i.e.60 juniors out of 800 and then 1 out of 1000.

so for both have to be a pair rule of multiplication works here i.e.
60/1000 * 1/800=3/40,000

Amit

thanks....