1) arbre pondéré :

- niveau 1:

      --------> V(3/4)

      --------> R(1/4)

- niveau 2 :

     V(3/4) --------> V(1/3)

                --------> R(2/3)

     R(1/4) --------> V(1/3)

                 --------> R(2/3)

2) a) P(VR)=3/4*2/3

                   =6/12

                   =1/2

b) P(VV)+P(RR)

   =3/4*1/3+1/4*2/3

   =3/12+2/12

   =5/12

c) p=1-P(VV)

      =1-3/4*1/3

      =1-1/4

      =3/4