Direct-Current Circuits: Series, Parallel, Kirchhoff’s Rules, and RC Circuits

In a circuit, three resistors $R_1$, $R_2$, and $R_3$ are connected in series. If $R_1 = 2\ \Omega$, $R_2 = 4\ \Omega$, and $R_3 = 6\ \Omega$, what is the equivalent resistance $R_{eq}$ of the combination?

Two resistors, $R_1 = 10\ \Omega$ and $R_2 = 20\ \Omega$, are connected in parallel. What is the equivalent resistance $R_{eq}$?

A circuit contains a battery of emf $\epsilon = 12\ \text{V}$ and three resistors in series: $R_1 = 2\ \Omega$, $R_2 = 3\ \Omega$, and $R_3 = 5\ \Omega$. What is the current $I$ in the circuit?