Question

physics
A car travels the first 30Km with a uniform speed of 60Km/hr and then next 30Km with a uniform speed of 40 Km/ hr. Calculate: i) the total time of journey, ii) the average speed of the car .

Answer 1

2 hrs I think so .but average speed is 20

Answer 2

Answer :         V1 = 60km/hr D1 = 30 km          T1 = ?          V2 = 40 km /hr D2 = 30 km          T2 = ?
Total Time = T1 + T2 
We know that ,           velocity = Distance /Time Therefore ,             V1 = D1/T1 or T1 = D1 / V1  => T1 = 30 km / 60 km /hr = 1/2 hr              V2 = D2/T2 or T2 = D2/ V2  => T2 = 30 km / 40 km /hr = 3/4 hr     Hence ,                Total Time = T1 + T2 = 1/2 + 3/4 = 5/4 hrs     we know that ,               Average Velocity = Total Distance / total time taken = 30 + 30 / 5/4                                          = 60/5/4 = 48 km /hrHence Average Velocity is 48km/hr and total time of journey = 5/4 hours.
My Short cut Method :-           When a body travels equal distance with different velocities V1 and V2 , then Average Velocity is Calculated as              Average Velocity = 2*V1*V2 / (V1 + V2 )             here , V1 = 60 km/hr and V2 = 40 km/hr      therefore ,        Average Velocity = 2*60*40 / (60+40 )= 4800/100 = 48km/hr 
Also we know that ,           Average Velocity =  Total Distance / Total Time Taken                      48 = 60 / Total Time Taken                      or Total Time Taken = 60/48 = 5/4 Hrs .

This was my Previous Solution !!