We would need to know the weight of the bag to find out how much there was of each ingredient, assuming these are the sole ingredients.
Suppose the weight of the bag of fertiliser is 19lb, then there would be 10lb of potash, 6lb of nitrogen compound, and 3lb of phosphorus. For weight w that is 10w/19 potash, 6w/19 nitrogen, 3w/19 phosphorus.