Class a has 25 pupils and class b has 21 pupils. both classes sit the same maths test. the mean score for class a is 78. the mean score for class b is 80. what is the mean score (rounded to 2 dp) in the maths test across both classes? just give the answer no calculations or anything .

Assume that there is a 11% rate of disk drive failure in a year. a. if all your computer data is stored on a hard disk drive with a copy stored on a second hard disk drive, what is the probability that during a year, you can avoid catastrophe with at least one working drive? b. if copies of all your computer data are stored on four independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?