If the question is asking which of the two will be used on one sale of $81,500 then use the 6.5% but if the question is asking what he earns if the first $50,000 is subject to the first interest rate, and the rest of it to the second then the answer will be as so; he earns 3.5% on his first $50,000, this is $1,750. He still has $31,500 that is earning 6.5%, which is $2,047.5. He earns a total of $3,797.50