18. A water tank is $\frac{5}{6}$ full. When 30 litres of water are removed from the tank, the tank is $\frac{4}{5}$ full. How much water does the tank hold when full?
A 180 litres
B 360 litres
C 540 litres
D 720 litres
E 900 litres
18. E Let the capacity of the tank be $x$ litres. Then $30=\frac{5 x}{6}-\frac{4 x}{5}=\frac{25 x-24 x}{30}=\frac{x}{30}$. So $x=30 \times 30=900$.