(i) A good is called 'inferior goods' when its demand falls with a rise in the income of a consumer and vice- versa.
(ii) For example, Jowar or Bajra for a poor person.
(iii) A good is inferior in a relative terms. It means, a good is inferior or normal is determined by the income level of a consumer.
(iv) When a consumer moves to higher income, he/she may consider some goods below their income status, and treats them as inferior.

(i) A good is called 'inferior goods' when its demand falls with a rise in the income of a consumer and vice- versa.
(ii) For example, Jowar or Bajra for a poor person.
(iii) A good is inferior in a relative terms. It means, a good is inferior or normal is determined by the income level of a consumer.
(iv) When a consumer moves to higher income, he/she may consider some goods below their income status, and treats them as inferior.

(i) Complementary goods are those which are useless in the absence of other goods and which are demanded jointly.

(ii) There would always exist an inverse relationship between price of complementary goods and demand for given commodity.
(iii) It means, with a rise in price of complementary goods, the demand for given commodity falls and vice-versa.
(iv) For example pen and refill, tea and sugar are complements to each other.

(i) Substitute goods are those goods which can be used in place of another goods and give the same satisfaction to a consumer.

(ii) There would always exist a direct relationship between the price of substitute goods and demand for given commodity.
(iii) It means with an increase in price of substitute goods, the demand for given commodity also rises and vice-versa.
(iv) For example, Pepsi and Coke, tea and coffee are substitute to each other.

It can be seen from the given demand functions, that 20 consumers do not want to demand the goods at any price greater than or equal to Rs.\(\frac { 10 }{ 3 } \). All of them demand only at a price of less than Rs.\(\frac { 10 }{ 3 } \). 
For a single consumer d[P] = 10 - 3P For 20 consumers, d _market[P] = [10 x 3P] x 20 = 20 - 60 P, for price \(\frac { 10 }{ 3 } \) and d_market[P]= 0 ...., for price> = \(\frac { 10 }{ 3 } \).