Final answer:
Zahn DDS should allocate $90 to the new tooth enhancement service.
Step-by-step explanation:
When allocating costs to different services, it is important to determine the stand-alone price of each service. In this case, Zahn DDS charges $890 for both the tooth restoration service and the new tooth enhancement service. However, the normal stand-alone price for the tooth restoration service is $800. We can calculate the amount that should be allocated to the new tooth enhancement service by subtracting the normal stand-alone price of the tooth restoration service from the total charges. So, the amount allocated to the new tooth enhancement service would be $890 - $800 = $90.