That's an infinite list. 81, 83, 85, 87, 89, 91 and so on.
82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
Place the values in ascending order: 76, 81, 89, 91, 97, 99. There are 2 middle numbers; 89 & 91. The median is the mean of the 2 middle numbers; or (89+91)/2 or 90.
89
The numbers 71, 73, 79, 83, 89 and 97 are prime.
71 73 79 83 89 97.
71 73 79 83 89 97.
71 73 79 83 89 97.
99-87 = 12
71 73 79 83 89 97
61, 67, 71, 73, 79, 83, 89, 97
If he wants a 90% average for the four tests, the least he can afford to get on thefourth one is 99%.(87 + 87 + 87 + 99) / 4 = 360/4 = 90%Good luck !