If a person travels 30km/h to the shops and back 20km/h what is the average speed?

Its not 25km/h, but 24km/hour why is this:s
HELP

Well eria, the answer has to be 25 km/h unless another speed is giving or the distance in which he/she goes to the store in ratio with the distance going home is 2:3

Assume the distance is 30 km between A and B
A - B = 1 hour
B - A = 1.5 hours

As such he took 2.5 hours to travel 60 km. The speed, therefore is 60/2.5 or 24 km per hour