Answer: 2,400 doctors used brand X aspirin.
Step-by-step explanation: The question notifies us that 3,600 total doctors were surveyed. This automatically lets us know that 6/6 doctors = 3,600 doctors. We can find the unit rate from this point by dividing 3,600 by 6. We will find that 600 doctors represents 1/6 doctors. We can multiply our unit rate by 4 to find that 4 out of 6 doctors, or 2,400 doctors, use brand X aspirin. 4/6 of 3,600 is 2,400, so 2,400 doctors used brand X aspirin. Hope this helps!