A mobile phone is available for Rs. 25,000 or Rs. 5,200 down payment, followed by 4 equal monthly instalments. If the rate of interest is 25% p.a. simple interest, calculate the amount of each instalment.

$$\eqalign{ & {\text{Cash payment}} = 25000 \cr & {\text{Down payment}} = 5200 \cr & {\text{Remaining}} = 19800 \cr & {\text{Installment}}\left( x \right) = 4 \cr} $$
\[\begin{array}{*{20}{c}} \Rightarrow &{19800}&{ - x} \\ {}&{19800}&{ - 2x} \\ {}&{\mathop {19800}\limits_{\_\_\_\_\_\_\_\_\_\_\_} }&{\mathop { - 3x}\limits_{\_\_\_\_\_\_\_\_\_} } \\ {}&{79200}&{ - 6x} \end{array}\]
$$\eqalign{ & \Rightarrow {\text{Interest}} = 4x - 19800 \cr & \Rightarrow \frac{{\left( {79200 - 6x} \right) \times 25}}{{12 \times 100}} = 4x - 19800 \cr & \Rightarrow 1920x - 50400 = 79200 - 60x \cr & \Rightarrow 1980x = 1029600 \cr & \Rightarrow x = 5200 \cr} $$