A sum of ₹12,000 was taken at simple interest at some rate. After four months, ₹6,000 more was added and the total principal was charged at double the earlier rate of interest. At the end of the year,if the total interest was ₹2,800, what was the initial rate of interest?

10%

Let initial rate = R% ,  Rate after 4 months = 2R%

We know ,

We know  ,

Simple Interest = \(\frac{Principal ×Rate × Time }{100}\)

\(\frac{12000 ×R× 4 }{12 ×100}\) + \(\frac{(12000+6000) ×2R× 8 }{12 ×100}\) = 2800

\(\frac{120 ×R }{3}\) + \(\frac{180 ×2R× 2 }{3 }\) = 2800

120R + 720R = 8400

840R = 8400

R = 10%