Story problem help? (I really suck at math)?

When astronauts Antoine Weber and Rachel Sun landed on the hypothetical planet Trifour they found the inhabitants had either 3or 4 extremities. Weber could not look at the inhabitants' faces so he just counted their extremities. He counted 173 extremities. Sun, on the other hand, could not look at their extremities so she just counted faces. She counted 51 faces. How many inhabitants had 3 extremities? How many were there that had 4 extremities?