At least 88.9% of the adults will have heights between 56.3 and 77.3 inches.
First, determine how many standard deviations from the mean the lower and upper limits are.
lower limit
66.8 - 56.3 = 10.5
10.5 / 3.5 = 3 standard deviations.
upper limit
77.3 - 66.8 = 10.5
10.5 / 3.5 = 3 standard deviations.
Chebyshev's theorem applies to all data sets and one of its claims is that at least 8/9 of the data points will lie within 3 standard deviations of the mean. Since you desire a percentage, do the division.
8/9 = .88888 = 88.9%
