If an atom of lithium has a mass number of 9, what is the number of neutrons?

6

Explanation:you find neutrons by subtracting the mass number from the proton for this example lithium's number of protons is 3 and mass number 9 so 9-3=6

b) mutualism

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< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >