Question 97149
Distance=rate*time. Here you have to add both Bob's & John's distances. 
270=3*b+3(b-20) [3*b is Bob's speed, 3(b-20)=John's speed]
270=3b+3b-60
6b=270+60
6b=330
b=330/6
b=55 mph. 
proof
270=3*55+3(55-20)
270=165+3*35
270=165+105
270=270