Distance covered in first 2 hours = (70 x 2) km = 140 km

Distance covered in next 2 hours = (80 x 2) km = 160 km

Remaining distance = 345 – (140 + 160) = 45 km.

Speed in the fifth hour = 90 km/hr

Time taken to cover 45 km  = (45/90) hr  =   (1/2)  hour 

Therefore Total time taken  =  2 + 2 +  (1/2)  =  4 (1/2)hrs