Your question doesn't make sense, it should be "What is six and seven eighths minus four" not six and seven eights, it makes it confusing.
Anywho,
(67/8) - (4) = (27/8)
Bah humbug.
Six eights.
187
Six eighths, or three fourths.
Converting all to eights, it gives three eights minus six eights, so the answer is minus three eights.
Expressed as a decimal, 6 2/3 - 4 7/8 = 1.7916 recurring (that is, 1.7916666..)
It is negative six and five eighths.
Zero.
3¼
5 5/8
5 5/8
Seven is the number of eights you are seeking.
Four times minus six is minus twenty four.