题目

Many people love going on a trip by plane, but an airport is usually far from the city. You have to arrive there and wait for hours for the plane to take off and it is often late. You can't open the window. You can't choose the food. Planes are fast, but it takes hours to get out of the airport and into the city.I like travelling by train. I think trains are safe. Train stations are usually in cities. When you are late for a train, you can catch another one. You can walk around in the train and open the windows. You can see many interesting things on your way. I know it takes a little more tim为了估计水塘中的鱼数，养鱼者首先从鱼塘中捕获30条鱼，在每条鱼身上做好记号后，把这些鱼放归鱼塘，再从鱼塘中打捞200条鱼，如果在这200条中有5条鱼是有记号的，则鱼塘中的鱼估计大约有________条．