A boy started for a school which is 12 km from his home by bicycle at a speed of 8 km/hr but after 15 minutes he remembers that he forgot one of the book at home, so he increases his speed and returns back to home and also reaches to school in time. The percentage increase in his speed is
Usual time to reach school=12/8=1.5 hrs
Distance covered in 15 min. at a speed of 8 km/hr=(15/60)*8=2 km
So, he comes back 2 km to home and again return to school at a speed of say 'x' km/hr.
Then, usual time of 1.5 hrs = 15 min. + Time taken for the journey of (2+12) km with increased speed.
1.5 = (1/4) + (14/x)
x=11.2
Therefore % increase in the speed = [(11.2 - 8)/8]*100 = 40%
Correct Option 7)