The addition of sodium chloride to pure water, causes the conductivity to

The addition of sodium chloride to pure water causes the conductivity to INCREASE. The more NaCl present in the water, the higher the conductivity is because NaCl is an electrolyte. Once dissolved in water, it becomes Na and Cl which are corpuscles that conduct electricity. 

So what you would do is go through and add 3 teaspoons(tsp) 6 times because of the 6 tablespoons(tbsp). which will give you an answer of 18. another way is to multiply 3x6=18.