Given:
Total distanced traveled is 200 miles.
speed is 60 mph, but as the congestion got bad, they had to slow to 10 mph.
Time = 6 hours.
Let, x be the travel time at 60 mph
The equation is,

It mean at the speed of 60 mph, the distance traveled is,

So, the distance traveled for the speed of 10 mph is,

Answer: The family traveled 32 miles at reduced speed.