For low is approximately 29 percent. Again imagine 1000 cabs. With a base rate of 15% green, 150 of them are green and 850 are blue. If the witness report is reliable 70% of the time, than there are .70 * 150 (105) ways to correctly report a cab as green. And there are .30 * 850 (255) ways to incorrectly report a cab as green, leading to a totala of 360 ways to report a cab as green. Of those 360 ways, 105 are correct reports. So, if there is a report of a cab as green then there is a 105/360 chance that the report is correct.