400
18 goats walk to the top of a hill to eat the grass. Some goats have horns and some do not. How many goats have horns? How many do not?
1 has horns, 17 do not; 17 have horns, 1 does not;
2 have horns, 16 do not; 16 have horns, 2 do not;
3 have horns, 15 do not; 15 have horns, 3 do not;
4 have horns, 14 do not; 14 have horns, 4 do not;
5 have horns, 13 do not; 13 have horns, 5 do not;
6 have horns, 12 do not; 12 have horns, 6 do not;
7 have horns, 11 do not; 11 have horns, 7 do not;
8 have horns, 10 do not; 10 have horns, 8 do not;
9 have horns, 9 do not