If force = mass x acceleration, and you're walking down the street one day and you see a 20kg rock that is accelerating at 20 m/s², then you can say to yourself:
"Self ! That rock has 20 kg of mass and it's accelerating at 20 m/s², and since force = mass x acceleration, the net force acting on the rock must be (20kg) x (20 m/s²) which is 400 Newtons. How strange that 400 N is not one of choices under the question!"
