Probability Help

The reading speed of a sixth grade student is normally distributed with a mean speed of 125 words per minute and a standard deviation of 24 words per minute.
a) what is the probability that a randomly selected sixth grade student reads less than 100 words per minute?
b) what is the probability that a randomly selected student reads more than 140 words per minute?
c) between 100 and 140 words per minute?

Convert to the standard normal distribution then read the values off your tables.

Thank you, but I don't know what the standard normal distribution formula is