P4 + 5O2 = P4O10 Balanced equation
moles P4 present = 23.9 g x 1 mole/123.88 g = 0.193 moles
moles O2 present = 20.8 g x 1 mol/32 g = 0.65 moles O2
From balanced equation, mole ratio O2 : P4 is 5:1. Is 0.65 moles O2 5x 0.193 moles? NO. You don't have enough O2.
O2 is limiting in this reaction.
theoretical moles of P4O10 = 0.65 moles O2 x 1 mole P4O10/5 moles O2 = 0.13 moles P4O10
mass of P4O10 produced = 0.13 moles x 283.9 g = 36.9 g