The ages of trees in a forest are normally distributed with a mean of 25 years and a standard deviation of 4. Approximately what percent of the trees are between 20 and 30 years old?
@Mathbreaker

Question

The ages of trees in a forest are normally distributed with a mean of 25 years and a standard deviation of 4. Approximately what percent of the trees are between 20 and 30 years old?
@Mathbreaker

anonymous · Answer

Calculate the z-scores for values = 20 and 30

z = (value - mean)/(standard deviation)

look up cumulative normal distribution and find the percentage between these two z-scores.

anonymous · Answer

do i do 20-25 or 20-30-25? @douglaswinslowcooper

anonymous · Answer

find cumulative dnormal distribution for 
z=(20-mean)/ s.d.
and for 
z = (30-man)/s.d.

and the difference between these two cumulative percentages is the answer you want, the percentage between 20 and 30.

If you had z=1 so P(1) = 84%
and z=0, so P(0) = 50%, then 
you would have 84-50=34% between thse two z scores.