Speed of a cyclist ( distance, time, speed...)

Question

A student rides her bike from her home to the store and back again. Her average speed to the store is 6.0 m/s, and coming back home it is 4.0 m/s. If the store is 840 m away from her home, what is the average speed of the student cyclist overall in riding to the store and back?

The cyclist's average speed (the answer) is 4.8 m/s, but I don't know how to get to that answer.

Answer 1

Time going is 840/6 = 140 sec

Time returning is 840/4 = 210 sec

Total time = 350 sec

Total distance = 1680 m

Average speed = 1680/350 = 4.8 m/sec