Solution

"Oh no, the answer is not 25 kmph. The answer, I think, is 24 kmph.

Let the distance to the shop be "D". Let the time to reach the shop at 20kmph be T1 and time to return home from the shop at 30 kmph be T2.

Speed = distance / time. Therefore, T1=D/20 and T2=D/30. Total distance travelled is D+D=2D. Total time required = T1+T2 = D/20+D/30. Therefore, total time = D/12.

Average speed = total distance / total time.

Therefore, average speed = 2D / (D/12).

So, average speed = (2D x 12) / D.

Thus, average speed = 24 kmph.

Some useful links for
your career:

• Union Public Service Commission - www.upsc.gov.in
• IIT-Kharagpur - www.iitkgp.ac.in
• Indian Statistical Institute - www.isical.ac.in
• Indian Institute of Technology Madras - www.iitm.ac.in
• Indian Institute of Management, Ahmedabad - www.iimahd.ernet.in
• Indian Institute of Mass Commission - www.iimc.nic.in
• IIT Bombay - www.iitb.ac.in
• Indian School of Mines, Dhanbad - www.ismdhanbad.ac.in
• Birla Institute of Technology, Ranchi - www.bitmesra.ac.in
• Central Institute of Fisheries Nautical and Engineering Training - www.cifnet.nic.in
• Indian Institute of Information Technology, Allahabad (Deemed University) - www.iiita.ac.in
• Central Marine Fisheries Research Institute, Kochi - www.cmfri.com
• Tata Institute of Social Sciences, Mumbai - www.tiss.edu