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Homework answers / question archive / In one town, 24% of all voters are Democrats
In one town, 24% of all voters are Democrats. If two voters are randomly selected for a survey, find the probability that they are both Democrats. Round to the nearest thousandth if necessary.
This problem can be solved quickly if we assume that the selection of the two voters are independent. This means that whether or not the first voter is a democrat has no bearing on whether or not the second is a Democrat. We don't know the size of the town, but if it is sufficiently large, then the probability of picking a Democrat will not decrease once the first voter's political party is chosen.
The reason why we have to make this distinction is because of the following argument. Consider an urn with four balls: two red and two blue. When we draw the first ball, the probability of getting a red or a blue ball is the same: 50%. However, once that ball is drawn, the probabilities of getting a red or a blue on the second ball changes, since there are only three balls in the urn instead of four, and the probability of each is 66% or 33%, depending on which was drawn.
Thus, if we pick a Democrat, there would be one fewer Democrat in the pool, so if the population of the town were small enough, the probability of the next Democrat could be less than 24%, and we don't have enough information to work with if this is the case. Hence, the assumption that the population of the town is large enough to ignore this had to be made.
When outcomes are independent, we can find the probability of multiple outcomes occurring together through multiplication.
P(2D)=0.24×0.24≈0.058
Thus, there is a 5.8% chance that two randomly selected voters are both Democrats in this town.