To help farmers and gardeners, commercial fertilizers have a big three-number "NPK" label on the bag that gives the amounts of three key plant nutrients: nitrogen, phosphorus and potassium. For example, a bag of fertilizer has by mass nitrogen, by mass potassium as potash , and by mass phosphorus as phosphate . Suppose a certain fertilizer has the following composition: ingredient percent by mass ammonium phosphate calcium nitrate urea inert ingredients What should the first (nitrogen) number on the bag label be? That is, calculate the percent by mass of nitrogen in this fertilizer. Round your answer to the nearest percent.

It seems your question is incomplete, as it is lacking the working values. An internet search showed me the full question, you can see it in the attached picture.

Let's say we have 100 g of the fertilizer.

45 g are of ammonium phosphate ( (NH₄)₃PO₄ ), of which:

45 g (NH₄)₃PO₄ * [tex]\frac{42 g N}{149g(NH_{4})_{3}PO_{4}}[/tex] = 12.7 g are of Nitrogen.

(We used the molar mass of ammonium phosphate in the denominator and three times the molar mass of nitrogen in the numerator)

18 g are of calcium nitrate (Ca(NO₃)₂), of which:

16 g Ca(NO₃)₂ *[tex]\frac{28gN}{164gCa(NO_{3})_{2}}[/tex] = 2.73 g are of Nitrogen.

So in total there are (12.7+2.73) 15.43 g of Nitrogen in 100 g of the fertilizer. So the percent by mass of nitrogen is 15.43%.

Rounding to the nearest percent the answer is 15.