Total ways =
C2 = 66
E3 = the event of the sum being 3
E6 = the event of the sum being 6
E9 = the event of the sum being 9
E12 = the event of the sum being 12
E15 = the event of the sum being 15
E18 = the event of the sum being 18
E21 = the event of the sum being 21
(sum maximum = 11 + 12 = 23)
[(Which is not divisible by 3)
(Highest number divisible by 3 is 21)
∴
n(E1) = 1,
n(E6) = 2,
n(E9) = 4 ,
n(E12) = 5 ,
n(E15) = 5 ,
n(E18) = 3 ,
n(E21) = 3]
∴ Required probability =
=
=