Q1.
Ans: 21 pesos
Q2.
Ans: The farmer served them with twentyseven potatoes.
For equalization:
No potato left for the first farmer.
Three potatoes left for the second farmer.
Five potatoes left for the third farmer.
This is because the potatoes were twentyseven in number and first farmer already consumed his share which is nine, the second farmer consumed six and the third farmer consumed only four.
Nine potatoes were meant for each farmer.
I hereby invite @fucho34 and @bentala for participation.