There are ten identical-looking urns that contain large numbers of red and blue beads. Eight of these urns contain a majority of blue beads (call them Blue Urns). The other two
urns contain a majority of red beads (call them Red Urns). The proportion of the majority
colour in each urn is 75%. You first select an urn at random and then draw four beads
from it without looking. Three of the beads are red and one is blue. What is the
probability that you chose a Blue Urn?