A tank, when one-fifth full, holds 75 litres of water. How many litres will it hold when full?

(1/5)x = 75

x = 75/(1/5)

15

To solve this problem, we can set up a proportion using the given information.

Let's assume that when the tank is full, it holds 'x' liters of water.

According to the given information, when the tank is one-fifth full, it holds 75 liters of water. This means that one-fifth of the tank's capacity is equal to 75 liters.

So, the proportion can be set up as:

(1/5) * x = 75

To find the value of 'x', we can solve this equation.

Multiplying both sides of the equation by 5, we get:

x = 75 * 5

x = 375

Therefore, the tank will hold 375 liters of water when it is full.