Problem. 21. We have a report in 2010 that indicates that Americans between the ages, the ages of the Americans between eight and 18 years it's been an average of new equals 7.5 hours per day using some sort of training device, and in this report, the Standard Division was it will 2.5 hours per day. It's required to find the probability of selecting an individual who uses electronic devices more than nine hours a day. Then for about A it's required to get the probability of X is greater than nine hours birthday. We can get this probability using the school instead. The score equals X minus mu, divided by sigma we have X here equals nine minus mu, which is given 7.5 Divided by sigma equals 2.5. Then the score for nine hours per day equals 4.6. By entering the table, we can get the probability that zit is greater than zero and smaller than 2.6. The tables we have gives the probability starting from zero to the specific is it which is also in six gives in this area the probability from the table equals four point oh 793 then took it. The probability that is greater than 4.6 or greater than X equals nine. Then the probability of X is greater than nine equals half, which is half the area of the normal distribution buying us the shaded area. Sorry, this value is not opened or 73 equals point two 57 Then the probability is half minus 0.2257 which equals 4.2743 which equals 27.43%. Then the probability to select an individual who uses electronic devices more than nine hours per day equals 27.43%. For Part B. We want to get the portion of the same age from 8 to 18 year old who spent between eight and 12 hours. She means we need to get the probability for Ex is greater than eight hours and smaller than 12 hours. Birthday, of course. Then, to answer this, we get to the scores the first season score for it. We use X as 88 hours per day, eight minus 7.5, divided by 2.5 gives open to and for the two we get X equals 12, then 12 minus 7.5. Divided by 2.5, it equals 1.8. Then we get the probability for each Z score from the tables for from zero that is greater than zero, said one. And between 1.6 Sorry. Here it's open to from the tables. It equals four point oh 793 And for the second school between 1.8, it equals 4.4 641 This means we have the normal distribution here is open to and here is one point. The first area here is for that equal 0.2. It's the probability is this value and we have another venue from zero to 1.8. We have poll this area. Then to get the area to get only this area, we'll get the probability for X. Between eight and 12 hours per day will be the shaded area with blue, which is opened 4641 minus the shaded area with black Oh, point Oh 793 then equals Oh, 0.38 for it or we can write it in percentage 38.48% and this is the final answer of body and this is the final answer of 40 and the problem.