3. If Peter walks along a moving escalator at his normal speed, it takes 40 seconds to finish riding on the escalator. If Peter walks at twice his normal speed, it takes 30 seconds to finish riding on the escalator. How many seconds does it take for Peter to finish riding on the escalator if he stands still on the escalator?
(1 mark)
If Peter walks at his normal speed on a moving escalator, it takes 40 seconds to finish riding on the escalator. If Peter walks at twice his normal speed on the escalator, it takes 30 seconds to finish riding on the escalator. If Peter stands still on the escalator, how many seconds does it take for him to finish riding on the escalator?
3. 60
3. Let \( x \) be the normal walking speed of Peter and \( u \) be the speed of the escalator. Since speed is inversely proportional to time taken, we have \(\frac{u+x}{u+2x}=\frac{30}{40}\), which gives \( u=2x \). Let the answer be \( t \) seconds. Then we have \(\frac{u}{u+x}=\frac{40}{t}\), so that \( t=\frac{40(u+x)}{u}=\frac{40(2x+x)}{2x}=60 \).