Average

Total amount of charity = $20 \times 4 = 80 Tk.$

Here, maximum 3 students can give = 25 Tk.

Total = $3 \times 25 = 75 Tk.$

Minimum amount contributed by one student = $80-75 =5 Tk.$

The sum of 15 consecutive integers will 88.

only when there are some positive and negative consecutive integers. 

So, we let, the numbers are $(x-7), (x-6), (x-5), (x-4) (x-3), (x-2) (x-1) x, (x + 1), (x+2), (x+3), (x+4), (x+5), (x+6) (x+7).$

According to question 

$x+(x+1)+(x+2)+(x+3)+(x+4)+(x+5)+(x+6)+(x+7)+(x-7) + (x -6) + (x -5) + (x - 4)+(x-3)+(x-2)+(x-1)=88$
$$\begin{gathered} \Rightarrow 15x+28-28 = 88 \\ \Rightarrow 15x = 88 \\ \therefore x=\frac{88}{15} \end{gathered}$$

The largest number is $(x+7)= \frac{88 }{15}+7=\frac{88+105}{15}=\frac{193}{15}$
But, here $\frac{193}{15}$ is not a integer. 

So the data given in the question may not be consistent.