Final answer:
The write-down adjustment is $42, calculated by subtracting the Medicare allowed amount of $78 from the provider's usual fee of $120.
Step-by-step explanation:
The write-down adjustment refers to the amount by which a healthcare provider reduces their usual and customary fee to the allowed amount determined by Medicare. In this example, the provider's usual fee for a service is $120, but Medicare's allowed amount is only $78. Thus, the write-down adjustment would be the difference between these two amounts.
To calculate this, we subtract the Medicare allowed amount from the provider's usual fee: $120 - $78 = $42. Therefore, the write-down adjustment that the provider has to make is $42.