Answer to Question 1
Sugar is a nonvolatile (does not form a gas), non-ionizing (because it is a molecular structure) solute (small particle dissolved in the solvent of water). A nonvolatile, non-ionizing solute will raise the boiling point of a liter of water by 0.52oC. Thus the addition of sugar to cooking water in boiling apples will raise the boiling point and result in the apples cooking in a shorter time than those cooked in plain water.
Salt is a nonvolatile (does not form a gas), ionizing (because it is made up of two ions, sodium and chloride), solute (small particle dissolved in the solvent of water). A nonvolatile, ionizing solute will raise the boiling point of a liter of water by 0.52oC per ion formed. Thus salt, which is composed of two ions, will raise the boiling point of a liter of water by 1.04oC. Salt will have double the effect of sugar and result in the apples cooking even more quickly than in the sugar mixture.
Answer to Question 2
Limited by the type of food - more if bland; by type of test - more if color rather than tasting and if simple score card; fewer samples if inexperienced judges.