Help with a finance problem please! (T-Bill)?

So she made 600 on the sale, or 600 / 38,400 = 1.56% if held for a year. If she held it for 2 months, that equates to 300 per month, or 3,600 in 12 months. On an annual basis then, 3,600 / 38,400 = 9.4%.