Original quantity of milk in the container = 40 litres

At first, the quantity of liquid removed from the container = 4 litres

Quantity of milk left in the container after first procedure = 40 – 4 = 36 litres

Let us try to write this quantity in terms of 40 litres to make it easier to do bigger calculations.

36 litres = $(40\;\left[ {1 - \FRAC{4}{{40}}} \right])$

Quantity of milk left in the container after SECOND procedure

$(\begin{array}{l} = \left\{ {40\;\left[ {1 - \frac{4}{{40}}} \right]} \right\} - \left\{ {\frac{4}{{40}}of\;40\;\left[ {1 - \frac{4}{{40}}} \right]} \right\}\\ = \left\{ {40\;\left[ {1 - \frac{4}{{40}}} \right]} \right\} \times \left[ {1 - \frac{4}{{40}}} \right]\\ = \left[ {40{{\left( {1 - \frac{4}{{40}}} \right)}^2}} \right]\END{array})$ 

Similarly, quantity of milk left in the container after third procedure

$(\begin{array}{l} = \left[ {40{{\left( {1 - \frac{4}{{40}}} \right)}^3}} \right]\\ = \left[ {40{{\left( {\frac{{40 - 4}}{{40}}} \right)}^3}} \right]\\ = \left[ {40{{\left( {\frac{{36}}{{40}}} \right)}^3}} \right]\\ = \left[ {40{{\left( {\frac{9}{{10}}} \right)}^3}} \right]\\ = \left[ {40 \times \frac{9}{{10}} \times \frac{9}{{10}} \times \frac{9}{{10}}} \right]\end{array})$ 

= 29.16 litres

∴ Milk now present in the container is 29.16 litres