s = number of students books
we know there are many student's books, so since there are "s", and each one weights 2.3 kgs, then all together must weight 2.3s, now the teacher's edition is just 1 alone so that's just 4.3 kgs.
[tex]\bf \stackrel{\stackrel{total}{weight}}{30}~~=~~\stackrel{students'}{2.3s}+\stackrel{teacher's}{4.3}+\stackrel{box's}{0.4}\implies 30=2.3s+4.7 \\\\\\ 30-4.7=2.3s\implies 25.3=2.3\implies \cfrac{25.3}{2.3}=s\implies 11=s[/tex]
