What is the common difference between successive terms in the sequence? 9, 2.5, –4, –10.5, –17

Note that you get 2.5 from 9 by subtracting 6.5.  You get -4 from 2.5 by subtracting 6.5.  Thus, the common diff. is -6.5.

Subtract the first s numbers::

9-2.5 = 6.5

Now subtract that from the 2nd number to see if it equals the next number in the sequence:

2.5 - 6.5 = -4

-4 - 6.5 = -10.5

The common difference is subtracting 6.5 ( -6.5)

The answer would be 27 because assuming that each package has 1 bar of soap and 1 tube of toothpaste, after 27 packages they would run out of toothpaste