Answer: his average speed on the second day is 35 miles per hour
Explanation:
Bill spends two days driving from point a to point b
Let x = his speed on the first day.
Let y = his speed on the second day
Speed = distance / time
Time = distance / speed
Total time spent on the trip is 18 hours
Let time that he spent on the first day be t hours
Time that he spent on the second day will be 18 - t
Bill drove 2 hours longer on the second day than on the first day. This means
t = 18 - t + 2
t+t = 18+2
2t = 20
t = 10
He spent 10 hours on the first day.
He spent 18-10 = 8hours on the second day.
Bill drove at an average speed of 5 miles per hour faster than he drove on the second day. This means that
x = y + 5 - - - - - - - -- 1
Distance travelled on the first day = speed on the first day × time spent. This becomes
10 × x = 10x miles
Distance travelled on the second day = speed on the second day × time spent. This becomes
8 × y = 8y miles
He drove a total of 680 miles over the course of the 18 hours. This means that
10x + 8y = 680 - - - - - - - - 2
Substituting equation 1 into equation 2, it becomes
10(y + 5) + 8y = 680
10y + 50 + 8y = 680
10y + 8y = 680 - 50
18y = 630
y = 630/18 = 35 miles per hour
x = y + 5 = 35 + 5
x = 40 miles per hour