So, to find the answer you'd first add up how many patients you have: 20+50+100 = 170. In a fraction your total would go on the bottom. Then you'd take how many patients are taking a real drug, which is represented by 20+50 = 70. This number would go on top.
So, the probability of a patient NOT getting a placebo is 70/170. Simplified would be 7/17.