<u>Answer:</u>
<u>For 1:</u> The first term is 10 and the common difference is 
<u>For 2:</u> The value of n is 27
<u>Step-by-step explanation:</u>
The n-th term of the progression is given as:

where,
is the first term, n is the number of terms and d is the common difference
The sum of n-th terms of the progression is given as:
![S_n=\frac{n}{2}[2a_1+(n-1)d]](https://tex.z-dn.net/?f=S_n%3D%5Cfrac%7Bn%7D%7B2%7D%5B2a_1%2B%28n-1%29d%5D)
where,
is the sum of nth terms
The 11th term of the progression:
.......(1)
Sum of first 4 numbers:
......(2)
Forming equations:

( × 8)
The equations become:


Solving above equations, we get:

Putting value in equation (1):
![25=a_1+10\frac{3}{2}\\\\a_1=[25-15]=10](https://tex.z-dn.net/?f=25%3Da_1%2B10%5Cfrac%7B3%7D%7B2%7D%5C%5C%5C%5Ca_1%3D%5B25-15%5D%3D10)
Hence, the first term is 10 and the common difference is 
The nth term is given as:

Solving the above equation:

Hence, the value of n is 27