Answer:
55 mph
Step-by-step explanation:
Let x represent Bob's speed. Then John's speed is x+10, and their respective times are found by ...
time = distance/speed
330/x = 390/(x+10) . . . . . . . the times are the same
330(x +10) = 390x . . . . . . . multiply by x(x+10)
3300 = 60x . . . . . . . . . . . . . subtract 330x
55 = x . . . . . . . . . . . . . . . . . . divide by the coefficient of x
Bob is driving at 55 miles per hour.
