Ashish is right, its just a mathematical derivation from basic formula. Here I am giving the derivation, hope it will help in better understanding -
Avg. Capital Employed = Opening capital + closing capital / 2
Now Opening capital = Closing capital - profits after tax (put this value in above formula)
Avg. Capital = (Closing Capital - Profits after tax) + Closing capital / 2
Avg. Capital = 2 closing capital - profits after tax / 2
Avg. Capital = Closing capital - half profits after tax
Thus above formula is directly derived from basic formula. You wont get opening capital in every question. There you can use this formula.