The average height of 20 students is 20 cm and the average height of 11 students is 20 cm . What is the average height of the 31 students?
Choose the correct choice from the following options
A) 40 cm  B) 20 cm  C) 30 cm  D) 25 cm  E) 35 cm

Rationale: the total height of the 31 students = ( 20 * 20 ) + ( 11 * 20 ) = 400 + 220 = 620 cm the required average height = 620 / 31 = 20 cm answer : b
Answer: B