 Hello and welcome to this session. In this session we are going to discuss the following question and the question says that, a person sold a CD player at a gain of 20% had he bought it for 25% less and sold it for $30 less he would have made a profit of 28%. Find the cost price of the CD player. We know that profit or gain is equal to selling price minus cost price. Also profit is equal to profit percent into cost price. With this key idea let us proceed with the solution. First let us suppose that the cost price of the CD player is $100. The gain in the first case is 20% as profit is equal to profit percent into CP. This implies in the first case gain is equal to 20% of $100 which is equal to 20 upon 100 into 100 that is equal to $20. Now as profit or gain is equal to SP minus CP therefore selling price is equal to profit plus cost price which will be equal to 20 plus $100 which is equal to $120. Now if he bought it for 25% less so first 25% of $100 is equal to $25. So the new CP will be $100 minus $25 which is equal to $75. Now in the second case he had made a profit of 28%. So in the second case gain is equal to 28 upon 100 into 75 which is equal to 4 into 7 is 28, 425 is 100, 25 into 3 is 75. So this is equal to $21. Again as selling price is equal to profit plus cost price therefore this is equal to 75 plus 21 which is equal to $96. So now the difference in the two selling prices is equal to $120 minus $96 which is equal to $24. So when difference is $24 cost price is equal to $100. This implies for the difference of $30 cost price is equal to 100 upon 24 into $30. 4 6 are 24, 4 into 25 is 100, 6 5s are $30 so this is equal to $125. Hence the cost price of the CD player is equal to $125 which is our answer. This completes our session hope you enjoyed this session.