I have the follwing question from my exam revision sheets:

"the lengths of a species of worm are normally distributed with a mean length of 21cm and a standard distribution of 1.5cm. find the probability that a worm selected at random is :

1. less than 23.4cm
2. more than 19.8cm
3. between 18-24cm"

since I do not know the total number of worms or the frequency of each worm length I cannot understand how to work this out apart from making the figures up then calculate it out.

any advice would be great, thanks