If $\frac{a+b}{2}, b, \frac{b+c}{2}$ are in HP, then which one of the following is correct?

If the sum of the first 9 terms of an AP is equal to sum of the first 11 terms, then what is the sum of the first 20 terms ?

If the 5^th term of an AP is $\frac{1}{10}$ and its 10^th term is $\frac{1}{5},$ then what is the sum of first 50 terms ?

What is the arithmetic mean of 50 terms of an AP with first term 4 and common difference 4 ?