A bridge and a potentiometer in code

You will write three functions for the measuring circuits of this course.

The idea. A meter bridge balances when $\dfrac{X}{R} = \dfrac{l}{100 - l}$. A potentiometer compares emfs by their balance lengths. A cell with a resistor across it has an internal resistance $r = R\,\dfrac{l_1 - l_2}{l_2}$.

Your tasks.

1. In unknown_resistance, return $X$ from the box $R$ and the balance point $l$ of a meter bridge, in ohm.
2. In second_emf, return the emf of a cell from the emf of a standard cell and the two balance lengths, in V.
3. In internal_resistance, return $r$ from $R$, the open balance length $l_1$ and the closed one $l_2$, in ohm.