Three capacitors of capacitances 2 μF, 3 μF and 6 μF are connected in series across a 12 V supply. What is the charge stored on the 6 μF capacitor?

For capacitors in series, $\frac{1}{C_T} = \frac{1}{2} + \frac{1}{3} + \frac{1}{6} = \frac{3+2+1}{6} = 1$, so $C_T = 1\,\mu\text{F}$. In a series circuit, all capacitors store the same charge: $Q = C_T \times V = 1 \times 12 = 12\,\mu\text{C}$. A common error is assuming capacitors in series store different charges, or computing charge as $Q = C_{individual} \times V_{supply}$, which gives the wrong answer.