方程 $\frac{3}{2}\left(a-\frac{5}{2}\right)x+\frac{1}{2}=1$ 和方程 $\frac{1.7-2x}{0.3}-1=\frac{0.8+x}{0.6}$ 的解相同，求a的值．

     Mr. Green lives in a big city with his wife and two children. One morning Mr. Green takes his new car and washes it when a friend comes. The friend stops and looks at the new car for a minute. Then he asks, "That's a new car. Is it yours? "
"Sometimes." Mr. Green answers.
   The friend is surprised (惊奇). "Sometimes? " he says. "What do you mean?" Mr. Green answers slowly, "Well, when there is a party in town, my daughter Jean drives it. When there is a football, it's my son Jim's turn. When I washes it and it looks very nice and clean, my wife Linda uses it. And when it needs cleaning. It's mine. Choose the correct answer.

为测量某金属块的密度：

某测量员是这样利用回声测距离的：他站在两平行峭壁间某一位置鸣枪，经过1．00s第一次听到回声，又经过0．50s再次听到回声。回声测距是利用了声波的____,已知声速为340m/s，则两峭壁垒森严间的距离为____m。