A group of 20 preschoolers were tested on reading readiness. The mean on this test is 30.9, with SD=2.08. What percentage of children scored between 25 and 28.5?
Use z-scores.
Find two z-scores using this formula:
z = (x - mean)/sd
Mean = 30.9
Sd = 2.08
Use 25 for x to determine first z-score.
Use 28.5 for x to determine second z-score.
Once you have both z-scores, check a z-table for the probability between the two scores. Convert to a percent.
I hope this will help get you started.