Tom drove from P to Q at constant speed 30 km/h and then drove from Q to P at constant speed X km/h. If the average speed of the whole journey is 45 km/h, then what is X?
+36915 Call QP = d
distance/ rate = time
d/30 = time one way d/ x = time return
total distance = 2d total distance / total time = 45
2d / (d/30 + d/x) = 45
2d / (( dx+30d)/30x) = 45
60 dx /(dx+ 30d) = 45
60x / (x+30) = 45
60 x = 45x + 1350
15x = 1350
x = 90 km/hr
