This seems to be straight forward but I can't seem to work it out:

The error made when a certain instrument is used to measure the body length of a butterfly of a particular species is known to be normally distributed with mean 0 and standard deviation 1mm. Calculate to 3d.p, the probability that the error made when the instrument is used once is numerically less than 0.4mm.

The answer given in my textbook is 0.311

Can someone explain why?

Thanks.