A train moving at an average speed of 72 km/h take 15 secs to completely cross a bridge that is 80m long. (i) Express 72 km/h in m/s. (ii) Find the length of the train in metre.

Tunajua kuwa $1 \text{ km} = 1000 \text{ m}$ na $1 \text{ saa} = 3600 \text{ sekunde}$.

$$72 \text{ km/h} = 72 \times \frac{1000 \text{ m}}{3600 \text{ s}}$$

$$72 \times \frac{1000}{3600} = 72 \times \frac{10}{36}$$ $$= 2 \times 10$$ $$= 20 \text{ m/s}$$

Kasi ya treni katika $m/s$ ni $\boxed{\mathbf{20 \text{ m/s}}}$.

Sehemu (ii): Pata urefu wa treni katika mita.

Wakati treni inavuka daraja kabisa, umbali wote uliosafiriwa ni jumla ya urefu wa daraja na urefu wa treni.
Kasi ($S$) = $20 \text{ m/s}$ (kutoka sehemu (i))
Muda ($T$) = $15 \text{ s}$
Umbali ($D$) = Kasi $\times$ Muda

$$D = 20 \text{ m/s} \times 15 \text{ s}$$ $$D = 300 \text{ m}$$

Umbali wote uliosafiriwa ($D$) = Urefu wa daraja ($L_B$) + Urefu wa treni ($L_T$).
Tunajua $L_B = 80 \text{ m}$.

$$300 \text{ m} = 80 \text{ m} + L_T$$

$$L_T = 300 \text{ m} - 80 \text{ m}$$ $$L_T = 220 \text{ m}$$

Urefu wa treni ni $\boxed{\mathbf{220 \text{ m}}}$.