at the beginning, it has :
10 blue, 10 yellow, 10 green, 10 brown, 10 red , 10 black...total of 60
students have removed : 3 black, 9 blue, 4 brown, 1 green, 5 red, and 8 yellow....total of 30
so now, there is : 1 blue, 2 yellow, 9 green, 6 brown, 5 red, 7 black...total of 30
ok, so u started with 60 and 30 were removed, leaving u with 30 marbles...and 2 of them are yellow.
so the probability of picking a yellow now is : 2/30 which reduces to
1/15 <== ur answer