First you need to check your calculations.
A container of 20ft carry a weight that is normally distributed with a mean of 5000 t and a standard deviation of 400 t. What percentage of container weight between 5400 and 5600 t?
This is where I reached, how should I conclude on the percentage?
For χ = 5400
Z_{5400} = (5400-5000)/400
Z_{5400} = 10
For χ = 5600
Z_{5600} = (5600-5000)/40
Z_{5600} = 15