1. **Problem Statement:** Calculate the square roots of the given numbers. 2. **Formula and Rules:** The square root of a number $x$ is a value $y$ such that $y^2 = x$. For positive numbers, the principal square root is the positive value. 3. **Calculations:** a) $\sqrt{25} = 5$ because $5^2 = 25$. $\sqrt{36} = 6$ because $6^2 = 36$. b) $\sqrt{49} = 7$ because $7^2 = 49$. $\sqrt{64} = 8$ because $8^2 = 64$. c) $\sqrt{0.16} = 0.4$ because $0.4^2 = 0.16$. $\sqrt{0.25} = 0.5$ because $0.5^2 = 0.25$. d) $\sqrt{0.64} = 0.8$ because $0.8^2 = 0.64$. $\sqrt{0.81} = 0.9$ because $0.9^2 = 0.81$. 4. **Summary:** The square roots are: a) 5, 6 b) 7, 8 c) 0.4, 0.5 d) 0.8, 0.9 This completes the calculation of all requested square roots.