Answer:
D
Step-by-step explanation:
Without doing any math, you can deduce that in a data set with the values 20, 30, and 50, the only possible number you could add that could bring the mean (average) to 50 would be D or 100. But this method will not always work so instead I'll show how to do it mathematically:
(20 + 30 + 50 + x)/4 = 50
20 + 30 + 50 + x = 200
100 + x = 200
x = 100