Last Updated on May 4, 2023
GMAT OFFICIAL GUIDE PS
Solution:
In this problem, there are only two events that could occur for the 3 coin flips. Either the coin will land on tails zero times, or the coin will land on tails at least one time. (Remember that the phrase “at least one time” means “one or more.”
Writing this in a probability statement yields:
P(landing on tails at least 1 time) + P(landing on tails zero times) = 1
Thus, we can say:
P(landing on tails at least 1 time) = 1 – P(landing on tails zero times)
Since we are tossing the coin 3 times, the outcome of zero tails in 3 tosses is the same as getting heads on all 3 tosses. We can calculate the probability of zero tails in 3 tosses as the probability of 3 heads in 3 tosses:
½ x ½ x ½ =1/8
Plugging this into our formula we have:
P(landing on tails at least 1 time) = 1 – 1/8
P(landing on tails at least 1 time) = 7/8
Answer: D
