4. One day, a truck driver drove through a tunnel and measured the time taken between the moment at which the truck started entering the tunnel and the moment at which the truck left the tunnel completely. The next day a container was added and the length of the truck was increased from $6 \mathrm{~m}$ to $12 \mathrm{~m}$. The driver reduced the speed by $20 \%$ and measured the time again. He found that the time taken was increased by half. Find the length of the tunnel (in metres).
一位貨櫃車司機鴐車穿過隧道時, 計筫了車子進入陋道直至整輛車離開隧道所需的時間。第
二天, 車子加上了一個貨樻, 使其總長度由 6 米變戌 12 米。司機把車速調低 $20 \%$, 並再次計算同一時間。他發現需時增加了一牛。求隧道的長度（以米爲單位）。
4. 24
4. We resort to the formula 'Distance $=$ Speed $\times$ Time'. Here 'distance' refers to the total length of the tunnel and the truck. When the speed is reduced by $20 \%$ and the time taken is increased by half, the distance must be increased by $(1-20 \%) \times(1+50 \%)-1=20 \%$. Hence, if we let the answer be $x \mathrm{~m}$, then we have $\frac{6+x}{12+x}=\frac{5}{6}$, which gives $x=24$.