The average of 21 numbers is 12. The average of the first 11 numbers is 9 and the average of the last 11 numbers is 15. What is the value of the middle number?
A trader buys 500 kgs of cotton for Rs 9,000. 10% of this cotton is spoiled due to rain. At what rate (Rs/Kg) should he sell the rest to earn 10% profit?