At the end of each minute, (1 -17.1%) = 82.9% of the element remains. At the end of 19 minutes, the quantity has been multiplied by that value 19 times, so the remaining amount is ...
Mean is the average of all the numbers so you add them all together and the amount of numbers you added together you divide that by the addition to get the mean. The mean for those 4 numbers is 11,299. Hope I helped!