Question 864853
Notice how 80 - 3*8 = 56 and 80 + 3*8 = 104. So 56 is 3 standard deviations below the mean and 104 is 3 standard deviations above the mean.



The interval we're focusing on is basically all the values that are within 3 standard deviations of the mean.



So we use the <a href="http://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule">empirical rule</a> and we see that roughly 99.7% of the population is within 3 standard deviations of the mean.



If you want that as a fraction, then this is where it gets tricky. The reason why is because 99.7% or 0.997 is an approximation and the approximation is very rough. 
A better approximation is 0.9973002, and an even better approximation is 0.997300203936740, and so on. 
This decimal expansion goes on forever without a predictable pattern. 


So essentially this number is NOT rational and cannot be written perfectly as a fraction. If you really want a fraction (it's not advisable really), then you could stick with 99.7% and write that as 99.7/100 = 997/1000. That's probably the most intuitive way to do it.