AccipiterQ wrote:
Molly is rolling a number cube with faces numbered 1 to 6 repeatedly. When she receives a 1, she will stop rolling the cube. What is the probability that Molly will roll the die less than 4 times before stopping?
A) 11/180
B) 125/216
C) 25/216
D) 91/216
E) 27/128
will post OE after a few attempts
The probability that Molly will roll the die less than 4 times before stopping is the sum of the following:
The probability that Molly will roll the die once: P=1/6 (she gets 1 on the first roll);
The probability that Molly will roll the die twice: P=5/6*1/6 (not 1, 1);
The probability that Molly will roll the die thrice: P=5/6*5/6*1/6 (not 1, not 1, 1).
P = 1/6 + 5/6*1/6 + 5/6*5/6*1/6 = 91/216.
Answer: D.