If I am interpreting the question correctly, I would say that we are mixing P4 with Cl2 to make PCl5.

It is important to balance the equation to get a mole ratio. This would be P4 + 10Cl2 ---> 4PCl5.

Therefore the mole ratio from P4 to PCl5, is 1 to 4, so for every 1 mole of P4 used, we get 4 moles of PCl5.

Now we need to calculate how many moles of P4 is in 20grams. The mass of P4 is 123.895 grams/mole. (4x the molar mass of P). Therefore, in 20 grams of P4, we have 20/123.895 = .1614 moles of P4.