The formation of SO3 using SO2 and O2 follows the below balanced reaction:

2 SO2 + O2 -> 2 SO3

If 10 moles of SO3 were formed, then we can conclude that an equal number of moles of SO2 (10 moles) were used. Similarly 5 moles of O2 were used.

Hence number of moles of unreacted SO2 = 20-10 = 10

Number of moles of unreacted O2 = 15-5 = 10