1st question
crossing the bridge for 3rd time and pay for devil - 0 pesos
Before crossing the bridge for 3rd time - 12 pesos
That means he had 24+ 12 after he crossed the bridge for 2nd time
Before crossing the bridge for 2nd time - 18 pesos
That means he had 18+24after he crossed the bridge for the 1st time
So he had 21 pesos originally

2nd question
3rd farmer ate 1/3 from the left potatoes, that means 8=2/3 So the 3rd farmer ate only 4 potatoes
That means 2nd farmer left 12 potatoes and he ate 1/3 left potatoes and that means 12=2/3 so 2nd farmer ate 6 potatoes. That says first farmer left 18 potatoes which is again equal to 2/3
That means 1st farmer ate 9 potatoes.
So there was 27 potatoes at the beginning 1st ate 9 and he can't eat any more. The 2nd farmer ate 6 so he can have 3more potatoes. The 3rd farmer ate 4 potatoes so he can eat 5more...

I invite @udayakumarage @oshadhi