Question 708174
That is a Proportion problem.  The idea is compare ratios.


The description tells about parts of chlorine liquid and parts of salt water; but the question only deals with chlorine liquid.  The pool requires 3 parts of chlorine liquid to 100 parts of pool water.


The ratio to use is 3:100, for parts chlorine liquid to parts pool water.  You can and usually will also use this as {{{(3/100)}}}, 
for (parts chlorine liquid)/(parts water).


Now for 4000 Liter pool, how much chlorine?  Trace back in meaning.  If the pool were 100 Liters, then how much chlorine liquid would it need?  We must assume volume here, so the 3/100 ratio is 3 liters chlorine liquid to 100 liters pool water.  Well how many times larger is 4000 liters?


NUMBERS IN EQUATION

{{{3/100= V/4000}}}
Find V.


{{{(3/100)*4000=(V/4000)*4000}}}
{{{highlight(120=V)}}}, Liters.