Distinguish two children from twice one child

In a country, men’s weights are normal with mean 70 kg and variance 18 kg², and children’s weights are normal with mean 30 kg and variance 9 kg². The weights of men and children are independent. Find the probability that

(i) a randomly selected man weighs more than two randomly selected children combined,

(ii) a randomly selected man weighs more than twice one randomly selected child. A certain HDB lift has maximum load 340 kg:

(iii) find the probability it is overloaded by 3 men and 4 children;

(iv) find the smallest a such that the probability that the average weight of three randomly chosen men exceeds a kg is less than 0.7.