reached speeds over 600miles an hour.speed为什么要加复数?

reached speeds over 600miles an hour.speed为什么要加复数? racing drivers have reached speeds over 600 miles an hour. Although Campbell reached a speed of over 304 miles per hour the winning car reached a speed of fourty miles an hour-much the winning car reached a speed of forty miles an hour much The winning car reached a speed of forty miles an hour--much It reached a speed of forty miles an hour.It _____ forty mil It reached a speed of forty miles an hour. It _____ forty mi The winning car reached a speed of forty miles an hour - muc 新概念中的题in a long drop,they reach speeds of 60 miles an hour a 求：the average speed over the 8 miles is 33 miles per hour.怎么 I often speed at 70 miles an hour如果把speed当名词用,那用at the speed