You don’t specify what we are supposed to be converting grams of pentane to, but I’m guessing it’s moles
First you need to find the molar mass of pentane.
Molar mass of C is 12.01 and molar mass of H is 1.008.
12.01*5 +1.007*12 = 60.05 + 12.08 = 72.13 grams C5H12 per mole
54.11/72.13 = 0.75 moles C5H12
So your answer is 0.75 moles C5H12