Probability of happening
1 / possible outcomes
1 / 4 = 25%
Happening in a row (3 times)
Probability ^ instances
0.25 ^ 3 = 0.0156 ~ 1.5%
NOT happening
1 - probability
1 - 0.25 = 0.75
NOT happening in a row (15 times)
Probability NOT ^ instances
(1 - 0.25) ^ 15
0.75 ^ 15 = 0.133 ~ 13%
Happening after NOT happening (15 times)
1 - (1 - 0.25) ^ 15
1 - 0.75 ^ 15
1 - 0.133 = 0.867 ~ 87%