Total probability = probability of getting 7 + probability of getting 8
favourable Cases for 7 - 2,2,2,1 ; 2,2,1,2 ; 2,1,2,2 ; 1,2,2,2
Probability of getting 7 = (0.5)(0.5)(0.5)(0.05)*4
Probability of getting 8 = (0.5)(0.5)(0.5)(0.5)
Total probability = 0.0250+0.0625 = 0.0875