turtle lifespan: mean of 180 yrs, st. dev. of 40 yrs

The life spans of a particular species of turtle are normally distributed with a mean of 180 years and a standard deviation of 40 years. What proportion of these turtles will live between 100 and 260 years?

What are your thoughts? What have you tried? How far have you gotten? Where are you stuck?

Please be complete. Thank you!

Eliz.