9th grade Algebra radicals. Worth 50 points

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?

The sales tax for an item was $8.40 and it cost $420 before tax. find the sales tax rate. write your answer as a percentage.

Acount went up from 450 to 600. what was the approximate percent increase?