A certain device is used to determine the sex of an unborn baby, but the device is not very reliable. If the fetus is truly a boy, the device says BOY with probability 0.8 (but mistakenly GIRL with probability 0.2). If the fetus is truly a girl, the device says GIRL with probability 0.7 (but mistakenly BOY with probability 0.3). Mrs. Jones and Mrs. Smith both use the device. For Mrs. Jones, it says GIRL. For Mrs. Smith, it says BOY. Who can be more sure about the sex of her unborn baby, Mrs. Jones or Mrs. Smith? Justify your answer using Bayes's rule.