Well let's call the fast bus b
That means the slow bus will be b-18
We know they travel in 5 hours, so multiply them both by 5
So 5b + 5b - 90 = 670
Simplify it down and collect like terms
10b = 580
Isolate b by dividing both by 10
b = 58
The fast bus travelled at 58mph
We know the slow bus travels 18mph slower than the fast one
58 - 18 = 40
The slow bus travels at 40mph
