Determining percentage of copper in brass
Well the question would had more to it. The question would have probably gone something like this. A sample of brass was dissolved with excess nitric acid. The $\ce{Cu^{2+}}$ ions are then reduced with excess iodide ions. Iodine is then titrated with thiosulfate (giving its concentration and volume used). Find the percentage of copper in brass. 

To do this question, calculate the amount of moles of thiosulfate used, using its concentration and volume. Then using equation 3, you can see the number of moles of iodine is half the number moles of thiosulfate. Then using equation 2, you can see that the number of moles of $\ce{Cu^{2+}}$ ions are twice as the number of moles of iodine and hence are equal to the number of moles of iodine. Finally, using equation 1, the number of moles of copper is equal to the number of moles of $\ce{Cu^{2+}}$ ions and hence are equal to the number of moles of thiosulfate. 

Now that we know the number of moles of copper, you can multiply by its molar mass to get its mass and then find the percentage of copper in brass.