21. Ellis's Eel Emporium contains a large tank holding three different types of eel: electric eels, moray eels and freshwater eels. A notice on the tank reads as follows:
All the eels are electric eels except 12
All the eels are moray eels except 14
All the eels are freshwater eels except 16
How many eels are in the tank?
A 42
B 33
С 24
D 21
E 20
21. D Let the number of electric eels be $x$, the number of moray eels be $y$ and the number of freshwater eels be $z$. The information on the notice tells us that $y+z=12, x+z=14$ and $x+y=16$. When you add these three equations, you obtain $2 x+2 y+2 z=42$ and hence $x+y+z=21$. Therefore the number of eels in the tank is 21 .