Barbara and her husband take a trip on a highway, a distance of 600 miles. They intend to average 60 m.p.h. and take 10 hours. However, they take 11 hours due to construction. When they encounter the construction, they reduce their speed to 40 m.p.h. and proceed at this speed until they reach their destination. How many miles from their starting point did they come to the construction?

Answer 1

distance at 60 mph --- x
distance at 40 mph --- 600-x

time at 60 mph = x/60
time at 40 = (600-x)/40

x/60 + (600-x/40 = 11
times 120

2x + 3(600-x) = 1320
2x+1800 - 2x = 1320
-x = -480
x=480

The encountered the construction after going 480 miles

check:
480/60 + 120/40 = 8 + 3 = 11