Aptitude - Aptitude-Average And Interests

If one student from group A is shifted to group B and that results in increase weight of both groups, then of the following is true ?

If the groups Aand B are combined and they are called as group D, then which of the following is true?

There match classes X,Y and z, take an algebra test. Tthe average score in class X is 83.The average score in class Y is 76. The average score in class Z is 85.The average score of all students in classes X and Y together is 79.The average score of all syudents in classes Y and Z together is 81. what is the average for all the three classes?

The average of 10 scores is 80. when the highest and the lowest scores are dropped, the average is 81. If highest score is 92, then lowest score will be ?

There are duplicating machines in an office. The fastest of the machines make P copies in 10 hours and slowest of the machines makes P copies in 11 hours. If each machines makes P copies, then averages time per machine cannot be ?