The average age of a class of 39 students is 15 years. If the age of the teacher is included, then the average increases by 3 months. Find the age of the teacher.
A milk merchant buys 50 litres of milk at the rate of Rs 40 per litre and mixes 5 litres of water in it. If he sells this mixture at the rate of Rs 42 per litre, then what is the profit percentage for the dealer?