题目

Mr. Green is a kind old man. He often helps people in need. And he’s kind of smart, too. One Sunday afternoon, he finished his lunch and went out for a walk with his granddaughter, Kate. In front of a park, they saw three beggars there. “Why do they ask for money in the street?” asked the little girl. “Because they are poor. ” answered Mr. Green. “But why don’t they go to work?” asked the little girl again. “Because they are too lazy.” Mr. Green smiled and said. Just then the girl asked, “Do you know who is the laziest of them, grandpa?” “That’s easy.” said Mr. Gr2004年12月28日，我国第一条城际铁路——合宁铁路(合肥至南京)正式开工建设．建成后，合肥至南京的铁路运行里程将由目前的312km缩短至154km，设计时速是现行时速的2.5倍，旅客列车运行时间将因此缩短约3.13h．求合宁铁路的设计时速．