Equivalent capacitance of two parallel capacitors $10\,\mu F$ and $6\,\mu F=\left(10+6\right)\mu F=16\,\mu F$ This $16\,\mu F$ capacitor is in series combination with $4\,\mu F$ capacitor,
$\therefore $ Equivalent capacitance of the entire combination $=\frac{16\times4}{16+4}=\frac{64}{20}=3.2\,\mu F$