Final answer:
John's insurer will indemnify him for a reduced loss of $250,000.
Step-by-step explanation:
To calculate the amount John's insurer will indemnify him, we need to subtract the reduced loss from the total loss. John's total loss is $300,000, but by renting the new building and equipment, he reduces the loss by $50,000. Therefore, the insurer will indemnify John for the reduced loss, which is $300,000 - $50,000 = $250,000.