Answer: It is Voidable
Step-by-step explanation:
Samuel took advantage of his fiduciary responsibility is taking care of Juan to unfairly influence him to sell him a piece of land at a price 35% below market price. Juan as an old man who is TOTALLY dependant on Samuel, felt he had no choice but to agree as failure to do so will lead to Samuel no longer taking care of him and this could be quite disadvantageous to him.
There was UNDUE INFLUENCE and Coercion in this scenario which means Voluntary consent was lacking.
For this reason, the contract can be voided.