There are an infinite list of different sets of 5 that do that.
If their mean is 80, then the only thing you know about them is that they add up to 400.
Here's one set:
78, 79, 80, 81, 82
Here's another one:
79, 81, -600, 400, 440.
The mean doesn't tell us anything about what the individual numbers must be.
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The range of a set of numbers is the difference between the highest and lowest values. In this case, the highest value is 100 and the lowest value is 80. Therefore, the range of the numbers 80, 90, 90, 95, and 100 is 20.
An interesting question. The answer is 80%. A number must end in 0 or 5 to be dividable by 5. From 100 to 109 (10 numbers in total), there will be 2 numbers that are dividable by 5. If I then consider the entire set from 100 to 999 is 900 numbers, you have 90 sets of 10 numbers, each set with 2 dividable number, or 180/900 = 2/10 =20%. If 20% are dividable, 80% aren't. Suppose I extended the question, and say my set is all whole positive numbers and zero with n digits or less, where I pick n randomly from 1 to a million. If from this set, a number is picked randomly, what is the chance that it is dividable by 5. Answer: 20%.
82.
-91
Multiply both numbers by 4 to get rid of the .25 and you have the equivalent question, "what fraction of 80 is 9?" The answer is 9/80