Granny, who has a mass of 50-kg, loves to roller skate and has taken her grandson with her to the rink. She whizzes
around the rink at 4 m/s and is suddenly confronted with her grandson, who has a mass of 30-kg, at rest directly
in her path. Rather than knock him over, she picks him up and continues in motion, while holding him, without
"braking". How fast are they traveling together?
