Teacher never explains his work

Question

In the reaction below, how many moles of A₂ will be used to produced 2.45 mol of XA?

a. Balance the chemical equation:   XY₃ +   A₂ →   XA +  Y₂A

b.    moles A₂

Answer 1

 2XY₃ +   A₂ →   2XA +  3Y₂A

Convert from moles to moles with coefficients:

(2.45mol XA/1) (1 mol A₂ / 2 mol XA)=1.225 mol A₂