Varun borrowed some money from karan at 12% simple interest then added an amount of his own to that and lent total to Charan at 18% simple interest.At end of year varun gained 672 .if varun lent 8000 to charan then how much did hi borrow from karan ?

6400

interest gained by charan(18% of 8000)- interest paid to varun(12% of x) is equal to 672. He got interest at 18% from charan but he also had to pay 12% interest to varun for the amount he borrowed. In the end he gained 672 i.e. even after paying varun he had money left from what he gained from charan. Hence the equation. 18% of 8000-12% of x=672. On solving it gives 6400