计算概率问题.题目如下.

计算概率问题.题目如下.
Two independent remote sensing devices, A and B, mounted on an airplane are used to determine the locations of disease tress in a large area of forest land. The detectability of device A is 0.8 ( that is, the probability that a group of diseased tress will be detected by device A is 0.8), whereas the detectability of device B is 0.9. However, when a group of diseased tress has been detected its location many not be pinpointed accurately by either device. Based on a detection from device A alone, the location can be accurately determined with probability 0.7, whereas the corresponding probability with device B alone is only 0.4. If the same group of diseased trees is detected by both devices , its location can be pinpointed with certainty. Determine the following.
(a) The probability that a group of diseased trees will be detected.
(b) The probability that a group of diseased trees will be detected by only one device
(c) The probability of accurately locating a group of diseased trees
a) 0.98 (b) 0.26 (c) 0.848
想问一下第三问怎么求出来的? 谢谢!感激不尽!