Which of the following numbers is irrational

Pls! does anybody know a shortcut for answering these types of questions in the future?

The brain volumes (cm cubed) of 50 brains vary from a low of 904 cm cubed to a high of 1488 cm cubed. use the range rule of thumb to estimate the standard deviation s and compare the result to the exact standard deviation of 175.5 cm cubed, assuming the estimate is accurate if it is within 15 cm cubed

The standard deviation for a set of data is 5.5. the mean is 265. what is the margin of error?