The monthly salaries of 30 employees of a firm are given below :
The firm gave bonus of Rs 10, 15, 20, 25, 30 and 35 for individuals in the respective salary group; exceeding Rs 60 but not exceeding Rs 75, exceeding Rs 75 but not exceeding Rs 90 and so on up to exceeding Rs 135 but not exceeding Rs 150. Find out the average bonus paid per employee.