Solution:
$$\eqalign{ {\text{Let C}}{\text{.P}}{\text{.}} = {\text{Rs}}{\text{. }}x. \cr Then,S.P. = {\text{Rs}}.\frac{{4x}}{3} \cr Gain = {\text{Rs}}.\left( {\frac{{4x}}{3} - x} \right) \cr \,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}\frac{x}{3} \cr \therefore {\text{Gain }}\% \cr = \left( {\frac{x}{3} \times \frac{1}{x} \times 100} \right)\% \cr = 33\frac{1}{3}\% \cr} $$

Solution:
$$\eqalign{ {\text{C}}{\text{.P}}{\text{. of 40 kg rice}} \cr = {\text{Rs}}.\left( {25 \times 32 + 15 \times 36} \right) \cr = {\text{Rs}}.\left( {800 + 540} \right) \cr = {\text{Rs}}.1340. \cr {\text{S}}{\text{.P}}{\text{. of 40 kg rice}} \cr = {\text{Rs}}.\left( {40 \times 40.20} \right) \cr = {\text{Rs}}{\text{. }}1608. \cr {\text{Profit}} = {\text{Rs}}.\left( {1608 - 1340} \right) \cr \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}268. \cr \therefore {\text{Profit }}\% = \left( {\frac{{268}}{{1340}} \times 100} \right)\% \cr \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 20\% \cr} $$

Solution:
Selling price of 8 articles = Cost price of 10 articles
8 Selling price = 10 Cost price
$$\eqalign{ \frac{{{\text{Selling price}}}}{{{\text{Cost price}}}} = \frac{{10}}{8} \cr {\text{Profit percentage}} \cr = \frac{2}{8} \times 100 = 25\% \cr} $$

Solution:
110% of 90% of 120% of A = 1188
$$\eqalign{ \Rightarrow \frac{{110}}{{100}} \times \frac{{90}}{{100}} \times \frac{{120}}{{100}}{\text{A}} = 1188 \cr \Rightarrow \frac{{1188}}{{1000}}{\text{A}} = 1188 \cr \Rightarrow {\text{A}} = 1000 \cr} $$
∴ A purchased it for Rs. (1000 - 110) = Rs. 890

Solution:
\[\begin{array}{*{20}{c}} {}{\text{I}}{{\text{II}}} \\ {{\text{Cost Price}} \to }{{{20}_{ \times 19}}}{{{20}_{ \times 21}}} \\ {{\text{Profit/Loss}} \to }{ + {1_{ \times 19}}}{ - {1_{ \times 21}}} \\ {{\text{Sales Price}} \to }{{{21}_{ \times 19}}}{{{19}_{ \times 21}}} \end{array}\]
21 × 19 unit → 9975
1 unit → 25
Loss = 2 unit = 2 × 25 = Rs. 50

Solution:
Let CP = Rs. 100
MP = Rs. 200
Gain = 15%
SP = 100 + 15% of 100 = Rs. 115
Discount = 200 - 115 = 85
% Discount = $$\frac{{85 \times 100}}{{200}}$$  = 42.5%

Solution:
$$\eqalign{ {\text{A}} \times \frac{{110}}{{100}} = 2860 \cr {\text{A}} = {\text{Rs}}{\text{.}}\,2600 \cr} $$