4. A botanist believes that the lengths of the branches on trees of a certain species can be modelled by a normal distribution.
When he measures the lengths of 500 branches, he finds 55 which are less than 30 cm long and 200 which are more than 90 cm long.
- Find the mean and the standard deviation of the lengths.
- In a sample of 1000 branches, how many would he expect to find with lengths greater than 1 metre?
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