Question: how many moles of o2 are needed to react completely with 3.0 moles of c2h6?
Answer: Multiply the given moles C2H6 by the mole ratio between O2 and C2H6 in the balanced equation, so that moles C2H6 cancel, leaving moles O2. ~11 moles of O2 are required to react completely with 3.2 moles of C2H6.