A week of rain

This week it rained on four of the seven days: Monday twelve millimetres, Tuesday eight, Thursday twenty and Sunday four.

Rain this week: $4$ of $7$ days. Mon $12$ mm, Tue $8$, Thu $20$, Sun $4$.

$R=\{\text{Mon},\text{Tue},\text{Thu},\text{Sun}\}$, $|R|=4$ of $7$. $r=(12,8,20,4)$ mm.

In all, forty-four millimetres fell, which is about six millimetres a day averaged over the week.

Total $=12+8+20+4=44$ mm; mean $=44/7\approx 6.3$ mm per day.

$\sum r=44$ mm, $\bar{r}=44/7\approx 6.3$ mm/day.

Last week only twenty millimetres fell, so this week brought more than twice as much rain.

Last week: $20$ mm. $44/20=2.2$: more than twice.

last week $=20\Rightarrow 44/20=2.2>2$.

The forecast gives a seventy per cent chance of rain tomorrow and twenty per cent the day after. The river stands thirty centimetres higher than a week ago; it rises about seven and a half centimetres for every rainy day.

Forecast: $P(B)=0.7$ tomorrow, $0.2$ the day after ($B$ = rain). River: $+30$ cm in a week, about $+7.5$ cm per rainy day.

$P(B_{+1})=0.7,P(B_{+2})=0.2$. river: $\Delta h=+30$ cm/week, $\approx +7.5$ cm per rainy day.