A cell phone is available for Rs.600 or for Rs.300 cash down payment together with Rs.360 to be paid after two months. Find the rate of interest charged under this scheme.

Price of the cell phone (without credit) = Rs. 600

Cash down payment = Rs. 300

Balance to be paid after 2 months = Rs. 360

The simple interest formula is:

I = $$\frac{P \times r \times t}{100}$$​​

Where:

I is the interest,

P is the principal amount,

r is the rate of interest per period,

t is the time in periods (months). 

In Rs. 600, 

Cash down payment Rs. 300 

remaining principle 300,

balanced paid 360 

Interest = 360 - 300 = 60 

60 = $$\frac{ 300 \times r \times 2}{100 \times 12}$$​  

r = $$\frac{60 \times 100 \times 12}{300 \times 2} = 120 \%$$ 

Thus, rate of interest is 120% per annum.