How many moles are in 15g of lithium?

There are 2.16 moles of Lithium in 15 grams. 

Look at the periodic table to find the molar mass of an element. Molar mass of Lithium is 6.9410 g/mol. 

Use dimensional analysis:
15g* (1mol/(6.9410g/mol))=answer
The grams cancel out leaving you with your number in mol units.
Multiply it out and you get 2.16 moles of Lithium. 

It changes food into energy that the organism can use.