Can someone help me understand this better by breaking down the thought process/rules behind the statements and how/why it should be $3500?

An insured has an HO-3, no endorsements, with following limits: A - $70,000, B - $7,000, C - $35,000,D- $14,000. The policy has a $500 deductible ($500 deductible for windstorm/hurricane). What is the amount payable (if any) when:" A windstorm caused extensive damage to the roof of the insured's residence, requiring replacement of a portion. Cost to replace was $5,000. Taking depreciation into account, the actual cash value was $3,000. At the time of the loss, the replacement value of the dwelling was $125,000"? $______ is payable. $5,000 less $500 deductible = $4,500,but for full replacement cost settlement, the insured needed not less than 80% of the full replacement value as the coverage A limit ($125,000 x 80% = $100,000).

Answer is $3,500.