Aplant fertilizer contains 15% by mass nitrogen (n). in a container of soluble plant food, there are 10.0 oz of fertilizer. how many grams of nitrogen are in the container?

The trick here is to convert the unit oz or ounce to grams. The ratio is 1 ounce is to 28.34 grms. This would mean 10 oz is equivalent to 283.50 grams. It is stated that the fertilizer is 15% by mass nitrogen. This means for every 100g of fertilizer 15 grams of that is nitrogen. Hence for every 283.50 grams of fertilizer 

283.50 * (15/100) = 42.52 grams is nitrogen. 

There are several information's already given in the question. The important thing is to convert ounce into grams to get to the right answer. We already know that 1 ounce is equal to 28.34 grams.
Then
10 ounce =10 * 28.34 grams
                = 283.40 grams
It has been given in the question that 15% nitrogen is present in the fertilizer. 
So
Amount of Nitrogen in
grams that is present in the fertilizer = 283.40 * (15/100) gm
                                                          = 4251/100 gm
                                                           = 42.51 gm
So 42.51 gm of nitrogen is present in the container. I hope the answer comes to your help.

The answer is true for your question.