Answer:
1.40 moles.
Explanation:
The balanced chemical equation for the reaction of Phosphorus and Oxygen is as follows -

In Part A, the oxygen was taken in excess. So Phosphorus will be the limiting reagent.
Since, 2 moles of
is formed by 4 moles of 
So, for 1.8 moles of
amount of required moles of
=
In Part B, the phosphorus was taken in excess so oxygen will be the limiting reagent.
Since, 2 moles of
is formed by 5 moles of oxygen
So, for 1.40 moles of
moles of
required = 
Thus as of now we have 3.60 moles of
and 3.50 moles of
.
As in the reaction of formation of
, oxygen is the limiting reagent.
So the moles of
formed by the 3.50 moles of oxygen will be
=
= 1.40 moles.