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find the concentration

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Hi there! 
 
To answer this question we have to consider the nature of the two solutes being mentioned. 
 
Glucose is a molecular compound, so when it is put in solution it doesn't dissociate. So if I put 30 molecules of glucose in water I will have 30 molecules of water floating in solution. 
 
NaCl is an ionic compound, so when it is put in solution it DOES dissociate into Na+ and Cl- ions. So, if I place 30 molecules of NaCl in water I end up having 30 Na+ and 30 Cl- ions in solution (for a grand total of 60 molecules in solution). Due to this difference between glucose and sodium chloride it is necessary we adjust the concentration of the NaCl so that it remains isotonic to the glucose solution. 
 
Since NaCl dissociates into two entities we would consider using a 0.15 M solution of NaCl because this would end up dissociating in solution giving us the target concentration of 0.30 M of ions.