My room

My room is four metres long and three metres wide, so it has twelve square metres of floor. The ceiling is two and a half metres high.

My room: $4\text{m}\times 3\text{m}=12{\text{m}}^2$ of floor, height $2.5$ m.

room $=4\times 3\times 2.5$ m. floor $=12{\text{m}}^2$.

Three things in it matter: a bed, a desk and a bookshelf. The bed takes a third of the floor; the desk and the shelf together take a sixth.

Three things matter: bed, desk, shelf. Bed $=\frac{1}{3}$ of the floor; desk $+$ shelf $=\frac{1}{6}$.

$\{B,D,S\}$ (bed, desk, shelf). $B=\frac{1}{3}\cdot 12=4{\text{m}}^2$; $D+S=\frac{1}{6}\cdot 12=2{\text{m}}^2$.

The window faces east, so the sun comes in from six until about ten in the morning. After that the room is in shade.

Window $\to$ east: sun from $6:00$ to $\approx 10:00$; after that, shade.

window $\to$ E. sun: $t\in [6:00,10:00]$; shade: $t>10:00$.

On the shelf there are forty books: thirty I have read, ten I have not. Every month I read two more, so in five months I will have read them all, if I buy none.

Shelf: $40$ books, $30$ read, $10$ not. $+2$ read per month $\Rightarrow$ all read in $10/2=5$ months, if I buy $0$.

$|K|=40$, $|K_r|=30$, $|K\setminus K_r|=10$. $+2$/month $\Rightarrow 10/2=5$ months (if $+0$ bought).