The average speed of a train running at a speed of $30 \, km/h$ during the first $100 \, km$,at $40 \, km/h$ during the second $100 \, km$,and at $50 \, km/h$ during the last $100 \, km$ is nearly (in $km/h$):

The average of $100$ observations was calculated as $35$. It was found later that one of the observations was misread as $83$ instead of $53$. The correct average is: (in $.7$)

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:

The sum of the ages of father and son at present is $33 \, \text{years}$. Two years ago, the product of their ages was $28 \, \text{years}$. What is the age of the father and the son (in $\text{years}$)?

$A$ student finds the average of ten $2$-digit numbers. While copying the numbers,by mistake,he writes one number with its digits interchanged. As a result,his answer is $1.8$ less than the correct answer. The difference between the digits of the number in which he made the mistake is:

The average age of Donald,his wife,and their two children is $23 \text{ years}$. His wife is $4 \text{ years}$ younger than Donald,and his wife was $24 \text{ years}$ old when his daughter was born. He was $32 \text{ years}$ old when his son was born. What is the average age of Donald and his daughter? (in $\text{years}$)