Question 1006314
The limits 64-83.5 are 1.5 standard deviations from the mean.  
1/1.5^2=4/9 outside maximum from Chebyshev's Inequality.
The minimum amount is 5/9 or 55.55% (repeat)
At least 90% are involved means 1/(sd)^2 must be equal to 0.10.  That makes SD^2 =10 and SD=sqrt 10 or 3.16 SDs.  That is 20.55 years on either side of the mean, so 90% of the population is included between 53.20 and 94.25 years.