Rounding to significant figures

Python's round(x, n) rounds to a number of decimal places. A chemist often needs a number of significant figures instead. You will write a function for it, and use the rules for products and sums.

The idea. The first significant digit of $x$ is at the place $\lfloor \log_{10} |x| \rfloor$: $0$ for units, $2$ for hundreds, $-3$ for thousandths. To keep $n$ figures, round to $n - 1 - \text{place}$ decimal places.

Your tasks.

1. Complete round_sf(x, n).
2. Set area to the area of a $3.15\ \text{cm}$ by $2.4\ \text{cm}$ card, with the right number of significant figures.
3. Set total to $14.62\ \text{g} + 3.4\ \text{g} + 0.385\ \text{g}$, with the right number of decimal places.