Answer:
Good style in solving such problems is to notice/(to observe) immediately from the condition that the difference
  between two combinations is  the cost of 2 cookies,  and it is equal to  $9 - $7 = $2.  So  2 cookies cost  $2.
  Then subtracting $2 dollars for 2 cookies from the Greg purchase, you got the cost of two bottles - it is equal to  $7 - $2 = $5.
  Answer.  A bottle of water costs  $2.50.
Step-by-step explanation: