speed problem

Leo needs to drive from Boston to Chicago. He planned to drive for 20 hours at an average speed of 50 mph. But, he drove at an average speed of 60 mph for the first 5 hours. During the remaining part of the trip, his average speed was 50 mph. How much earlier did he arrive in Chicago than he usually planned

Well..... using the original calculation.......the total   distance  must  be  20 * 50     = 1000 miles

So   he traveled  60 * 5   = 300 miles   and the  distance he  had left to travel  = 1000 - 300 = 700 miles

So

Distance  / Rate  = Time

So

700 / 50   =    14   hrs

So....his total travel  time =   5  + 14   = 19  hrs

So.........he arrived one hour earlier than he originally planned