Functions Modeling Change: A Preparation for Calculus, 5th Edition

After $5$ hours, the total will be halved, so the value is $\frac{1}{2}$. After 10 hours, another half will be depleted, so $\frac{1}{2}\cdot \frac{1}{2}=\frac{1}{4}$. Thus, $\frac{1}{4}$ of the substance will remain after $10$ hours, not $25$ hours.