7.4. Yura was walking down the road and met a tractor pulling a long pipe. Yura decided to measure the length of the pipe. For this, he walked along it "against the direction of the tractor" and counted 20 steps. After that, he walked along the pipe "in the direction of the tractor" and counted 140 steps. Knowing that his step is 1 m, Yura was able to find the length of the pipe. What is its length? Justify your answer.
Answer: 35 m.

Let's say that in the time it takes Yura to make 20 steps, the train travels $x$ meters. Denoting the length of the train by $L$, we get: $20=L-x, 140=L+7 x$. From this, $L=(140+20 \cdot 7): 8=35$.