problem: Dr. Zaius invests $\$10,000$ in a CD with an annual interest rate of $4\%$ that compounds semi-annually (twice a year).  After six months, he rolls over the CD into another CD with an annual interest rate of $5\%$ that also compounds semi-annually.  After six months in the second CD, how much does Dr. Zaius have, in dollars?
solution: The first CD compounds at a rate of $4/2 = 2$ percent for the first six months, so Dr. Zaius has $10000 \cdot 1.02 = 10200$ dollars.  The second CD compounds at a rate of $5/2 = 2.5$ percent for the next six months, so Dr. Zaius then has $10200 \cdot 1.025 = \boxed{10455}$ dollars.