2 tench fish were added.
in total there are 70 after the 10 added. the probability of getting a carp is 1 half so divide 70 by 2 to see how many carp there are. 70/2 = 35. so right now there are 35 carp and 35 tench. Now, before the 10 were added, there was 60 fish and 27 were carp. So subtract 27 from 60 to know how many tech fish there was. 60 - 27 = 33. There was 33 tech fish and from 33 to 35 goes 2. So 2 tench fish were added.
