Chebychev's Inequality & Central Limit Theorem

[andrew2786]

Hi,
Here is the problem I have, any help would be greatly appreciated:

In an experiment to determine the mean yield per acre of a new turnip seed, n plots of one acre each are planted with the new seed. Assume that 10cwt per acre is the population standard deviation.

Determine a sample size, n, sufficient to ensure a probability of at least 0.80 that the sample mean (average yield of the sample plots) will not differ from the population mean by more than 2cwt per acre using

a) Chebychev's Inequality and
b) the Central Limit Theorem

Why are the two results different?

Please help, I just don't know how to do this!

Many thanks

Andy