Let x be the number of monsters:
a) number of 0 horn = 2x/5
b) number of 1 horn = x/7
c) number of 2 horns = x/3
The 26 remaining monsters are:
(x) - (2x/5 + x/7 + x/3) = 26, solve for x,
x - 2x/5 - x/7 - x/3 = 26, reduce to same denominator:(105)
x - 92x/105 = 2730/105
and x = 210 monsters