.
After 1-st transfer,
With the probability
=
you will get (7+1 blue, 3 green) = (8 blue,3 green) in the bag
(call it "case A". It is the case when a blue jelly was transferred);
with the probability
=
you will get (7 blue, 3+1 green) = (7 blue,4 green) in the bag
(call it "case B". It is the case when a green jelly was transferred).
After the selection from the bag,
the probability to get a green jelly from the bag is
=
in case A, and
the probability to get a green jelly from the bag is
=
in case B.
Thus the probability to get a green jelly from the bag is
P(green) =
+
=
+
=
=
.
Now use the formula for conditional probability to get the answer to the problem's question
P(green at the first transfer) =
=
=
=
=
. ANSWER
Completed and solved.