Hi, I'm taking a distance learning course in statistics so there are a few concepts that are not clear to me. First I was wondering if it was possible to add up Z scores. I have a problem in which two students have two different z scores on two exams. Peter has 1,69 on the first one and -0,13 on the second, and Mary has -0,21 on the first one and 1,07 on the second one. Can I add up the Z scores to say that Peter is better? My second interrogation is with this problem. Two detectors have a probability = to 0,7 of detecting an object. A clerck puts an object under both machines simultaneously. What is the probability that one machine or the other detects it? So I did MxN to get all the possibilities, =100 and then 7x7 for the detections. I got p=49%, but when I do a tree of all the possibilities, I get 91%. So I'm wondering which way should I solve this problem?