At February 15, 2017, Brent filed his 2016 income tax return (Due April 15, 2017), and he paid a tax of $15,000 at that time. on June 10, 2018, he filed an amended 2016 return showing an additional $3,000 of tax which was then paid. In 2019, Brent found that he should claim a refund of $6,000 because he failed to take some deductions to which he was entitled. What amount can he recover if he files the claim for refund on March 14, 2020? What if he files the claim on May 15, 2020?