how many liters of water containing a sodium concentrate of 0.050% sodium by mass must be ingested to consume more than 2.4g of sodium per...

This problem involves percent concentration by
mass. 

You have a solution that is 99.95% water and 0.050%
sodium by mass.  

First, you need to weigh a liter of your
solution.  Let’s say your solution weighs
1000g. 

1000*0.9995=999.50g of water per
liter

1000*0.00050=0.50g of sodium per
liter 

To find the number of liters you need to consume to
ingest at least 2.4g of sodium, simply divide 2.4 by 0.5 to get 4.8 liters of
solution.

When looking to create a solution, use this
formula for percent concentration by mass: mass solute/mass
solutionX100.

Hope this helps!