I have no "real" answer, because the river may curve...
Fun answer: farmer Ed clearly does not realize you can Not create a rectangular plot that borders a river unless it is artificial and instead of dirt, it would Need to be concrete to ensure a constant straight edge; although even in that case, it would no longer Be a river! It would be a mote or channel. So this problem is unsolvable at his current standards.
There are 37 jelly beans so p for the first one is 27/37. That leaves 36 in the bag. So p for the second jelly bean is 10/36=5/18. The combine probability is 27/37×5/18=3/37×5/2=15/74.
