$$TR = Rs. 5000, , P =AR = Rs. 50$$ $$TR = P imes Q$$ $$Q = dfrac {TR}{P}$$      $$= dfrac{5000}{50}= 100$$ When $$TR = RS. 4500, ,P (=AR) = Rs. 90$$ $$Q= dfrac{4500}{90}= 50$$ Market demand falls by $$(50-100) units = 50 units$$.