A farmer is selling watermelons on the market. He knows from experience that the average weight of his watermelons is 7.8kg and the standard deviation is 3.1kg. On a given day, he wants to take at least 350kg watermelon to sell. He packs some randomly chosen watermelons. Find the smallest number of watermelons he should take so that the probability that he has at least 350kg watermelon to sell is at least 0.99.