Graham purchased a bag of fertilizer at the hardware store to improve the state of his front lawn. he noticed three letters on the bag: npk. what do those letters stand for?

Nitrogen (N) , Phosphorus(P)  and Potassium (K)

Explanation:

In the constituents section of a fertilizer bag, NPK along with some numerical values such as 10-15-20 etc is mentioned.  

This NPK signifies the constituents of the fertilizer where N stands for Nitrogen, P stands for Phosphorus and and K stands for Potassium.  

Also the number such as 10-15-20 signifies the percentage amount of NPK with in the fertilizer such as a fertilizer bag of mark 10-15-20 contains 10 % nitrogen, 15 % phosphorus and 20% potassium.  

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