A car travel to a place 150 km away at an average speed of 50 km/hr and returns it 30 km/hr. What is the average speed of the whole journey?

The average of the squares of first ten natural numbers is 

If the average marks obtained by Balu, Benjamin and Babur is 80 and the average marks of Benjamin and Babur is 83, find marks obtained by Balu.

The average of four consecutive even numbers is 15. The 2nd highest number is: 

30 pens and 75 pencils altogether were purchased for Rs .510. If the average price of a pencil was Rs. 2, what was the average price of a pen? 

The average age of 30 students of a class is 15 years. A 20-year-old student left the class and replaced by two new students, whose age difference is 5 years. If the average age of all students is 15 years, then what will be ratio of young student to elder students.