Dear aspirant !

Hope you are doing great ! You should know the formula first and that is n =N/Na where n is the no . Of moles which is equal to given mass / molar mass and N is the no. Of molecules asked in the question and Na is avogadro number which is a constant . Compare both given mass / molar mass =N/ Na , keep the N one side put all the values ,and you will get lithium as your answer .

Hope it helps you !

Thank you