If 3.5 mL of distilled water is added to 1.5 mL of 0.280 M CoCl2 solution, what is the concentration of the resulting solution
You are diluting it (3.5+1.5)/1.5=10/3=3.33 times
That means, you now have a solution of .280*1/3.33 M
That means, you now have a solution of .280*1/3.33 M