A biker goes out for a ride and begins riding North at 18 km/h for 200 minutes, then the driver turns around and rides at 12 km/h South for another 50 minutes.

Explanation:

18 km / h

= 300 m / min

12 km / h = 200 m / min

distance travelled in 200 minutes = 300 x 200 = 60000 m

distance travelled in 50 minute in return journey = 200 x 50 = 10000 m

total distance travelled = 70000 m

total time = 250 minute

speed = 70000 / 250

= 280 m / min

= 16.8 km / h

Total displacement = 60000 - 10000 = 50000 m

total time = 250 min

velocity = 50000 / 250

= 200 m / min

= 12 km / h

found in the nucleus

has mass of one amu