An oil tank is 1/3 full. when 500 litres are added, the tank is 3/4 full. find the capacity of the tank in show working !

X - volume when tank is 1/3 full
y - volume of entire tank

A) x/y = 1/3  (or 3x = y)
B) [(x + 500 liters) / y] = 3/4
Substituting A) into B)
B) [(x + 500 liters) / 3x] = 3/4
B) (x + 500 liters) = 9x / 4
4x + 2,000 = 9x
2,000 = 5x
x = 400

(400+500)/y = 3/4
3y = 1,600 + 2,000
y = 1,200

Double Check
when tank is 1/3 full  = x/y = 400 / 1,200 = 1/3

After adding 500 liters
900 / 1,200 = 3/4