A parallel plate capacitor with air between the plate has a capacitance of \( 15 \, \text{pF} \). The separation between the plate becomes twice and the space between them is filled with a medium of dielectric constant \( 3.5 \). Then the capacitance becomes \( \frac{x}{4} \, \text{pF} \). The value of \( x \) is ________ .
