averages Questions

Out of four numbers, the average of the first three is $15$ and that of the last three is $16$. If the last number is $19$, then the first number is:

A tabulator while calculating the average marks of $100\text{ students}$ in an examination, by mistake enters $6.8$, instead of $8.6$ and obtains the average as $5.8$. Then the actual average marks of those students is:

A college has raised $75\%$ of the amount it needs for a new building by receiving an average donation of \ 600$fromthepeoplealreadysolicited.Thepeoplealreadysolicitedrepresent$ 60\%$ of the people the college will ask for donations. If the college is to raise exactly the amount needed for the new building, what would be the average donation from the remaining people to be solicited?

The average per day income of A, B and C is $\text{Rs. }45$. If the average per day income of A and B is $\text{Rs. }40$ and that of B and C is $\text{Rs. }43$, then the per day income of B is: