Solve for Ca in the equation: (Ca 32.76) / (0.1 25) = 1/2. Round to four significant figures.

$$0.1 \times 25 = 2.5$$

Substitute this value back into the equation:

$$\frac{Ca \times 32.76}{2.5} = \frac{1}{2}$$

Multiply both sides by 2.5 and by 2:

$$2 \times (Ca \times 32.76) = 1 \times 2.5$$ $$Ca \times (2 \times 32.76) = 2.5$$ $$Ca \times 65.52 = 2.5$$

$$Ca \approx 0.038156288$$

Rounding to four significant figures:

$$Ca \approx 0.03816$$

The value of $Ca$ is:

$$\mathbf{0.03816}$$