My bicycle

My bicycle has two wheels, each seventy centimetres across, so one turn of a wheel carries me about two metres and twenty centimetres.

My bicycle: $2$ wheels, diameter $70$ cm. One turn $=\pi \times 0.7\approx 2.2$ m.

wheels: $n=2$, $d=70$ cm, $C=\pi d\approx 2.2$ m.

It has three gears. In the lowest, one turn of the pedals turns the wheel twice; in the highest, four times; the middle one gives three.

$3$ gears: $1$ turn of the pedals $=2$, $3$ or $4$ turns of the wheel.

gears $g\in \{2,3,4\}$: $1$ pedal turn $=g$ wheel turns.

On flat ground I pedal about sixty turns a minute in the top gear, which is roughly five hundred and thirty metres a minute, or thirty-two kilometres an hour.

Flat ground, top gear: $\approx 60$ pedal turns a minute $\Rightarrow 60\times 4\times 2.2\approx 530$ m/min $\approx 32$ km/h.

flat, $g=4$, $60$ /min: $v=60\cdot 4\cdot 2.2\approx 530$ m/min $\approx 32$ km/h.

It weighs twelve kilos, a bit less than a quarter of me. It is blue where the paint is left, and the bell rings once for a friend and twice for a danger.

Weight $12$ kg, a bit less than $\frac{1}{4}$ of me. Blue where the paint is left. Bell: $1$ ring $=$ friend, $2$ rings $=$ danger.

$m=12$ kg $<\frac{1}{4}$ me. blue (where paint $\exists$). bell$(1)=$ friend, bell$(2)=$ danger.

School is four kilometres away. On the bicycle that is seven and a half minutes; on foot it is fifty. Every schoolday the bicycle gives me back more than an hour.

School: $4$ km. Bicycle $=4/32$ h $=7.5$ min; on foot $50$ min. Each schoolday it gives me back $2\times (50-7.5)=85$ min $>1$ h.

school: $4$ km. bike: $4/32$ h $=7.5^{\prime }$; foot: $50^{\prime }$. $\forall$ schoolday: $\Delta t=2(50-7.5)=85^{\prime }>1$ h.