Astudent pulls a 60.-newton sled with a force
having a magnitude of 20. newtons. what is the
magnitude of the force that the sled exerts on the
student?
(1) 20. n (3) 60. n
(2) 40. n (4) 80. n

20 N

Newtons third law states that any action (pulling the sled) has an equal and opposite reaction (the sled pulling back on you).
Think about pulling a sled on really slippery ice, you would pull yourself toward the sled just as much as the sled toward you.