1. x = number of ghosts

2. y = number of goblins

3. z = number of ghouls

4. x+y+z = 51

5. x = 0.5y

6. Number of ghouls on Saturday: (z/3)

7. Number of goblins on Saturday: y+11

8. Number of ghosts on Saturday: x+(2z/3)-11

9. Ratio of ghouls to goblins on Saturday: (z/3)/(y+11) = 1/3

10. Simplifying the above equation: z = y+11

11. So now we have three equations and three unknowns:

· x+y+z =51

· x = 0.5y

· z = y + 11

12. Using substitution method:

· 0.5y + y + z = 51

· 0.5y + y + y +11 =51

· 2.5y = 40

· y = 16

· x = 8

· z = 27