2023 AMC 10A Problems/Problem 7
Revision as of 16:33, 9 November 2023 by Walmartbrian (talk | contribs)
Janet rolls a standard 6-sided die 4 times and keeps a running total of the numbers she rolls. What is the probability that at some point, her running total will equal 3?
Solution 1
There are 3 cases where the running total will equal 3; one roll; two rolls; or three rolls:
Case 1: The chance of rolling a running total of 3 in one roll is 1/6.
Case 2: The chance of rolling a running total of 3 in two rolls is 1/6 * 1/6 * 2 since the dice rolls are a 2 and a 1 and vice versa.
Case 3: The chance of rolling a running total of 3 in three rolls is 1/6 * 1/6 * 1/6 since the dice values would have to be three ones.
Using the rule of sum, 1/6 + 1/18 + 1/216 = 49/216.
