advanced maths. need solution

The heights, H metres, of teak trees in a large plantation can be modelled by a normal distribution with a mean of 25 metres and a standard deviation of O metres. It is known that immature trees have a height of 18 metres or less. 24.2% of the plantation consists of immature trees. The remainder of the plantation consists of mature trees. (a) Show that o = 10.00, correct to 2 decimal places. (b) Find the probability that a randomly selected tree has a height between 23 and 30 metres