An aeroplane flies along the four sides of a square field at speeds of $200,400,600$ and $800\, km / hr$. The average speed of the plane in the flight around the field in $km / hr$ is:

Find average of all prime numbers between $60$ and $90.$

With an average speed of $40\, km / h ,$ a train reaches its destination on time. If it goes with an average speed of $35 \,Km / h ,$ it reaches late by $15$ minutes. The total journey is (in $km$)

The average of five numbers is $27 .$ If one number is excluded, the average becomes $25 .$ The excluded number is

A librarian purchased $50$ $story-books$ for his library. But he saw that he could get $14$ more books by spending $Rs.\, 76$ more and average price per book would be reduced by $Rs.\,1.$ The average price (in $Rs.$) of each book he bought, was

In a class, the average height of $35$ girls was measured $160 \,cm$. Later, on it was discovered that height of one of the girls was misread as $144\, cm ,$ while her actual height was $104 \,cm.$ What was the actual average height of the girls in the class? (in $cm$) (rounded off to two digits after decimal)