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, phosphorus, potassium

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.