Using the Summation formula for this sequence,

what is S12 for 4 + 8 + 16+ ...

Hi, it will be really easy if you start out by plotting a tree diagram and tracing out the probabilities. Then you simply have to multiply the probabilities of placebo and improvement. 

80% Medication 76% improvement
                                 24% no improvement
|
20%Placebo 62% no improvement
                           38% improvement

Solving for P(placebo and improvement) = .20 x .38 = 0.076 = 7.6%