Put the weight of 70 at the same time as 20, then add salt to one side of 20 and add it to balance, which is 50g, and the remaining salt is 90 g, just do it once.

If the title is correct,

(1) is to divide 140g into two sides, with 70g on each side.

(2) Weigh 9g with two weights, and there are three piles at this time. They are 70g, 6 1g and 9g respectively.

(3) Weigh out 1 1g salt with 9g salt +2g weight, then there are four piles, 70g, 50g, 1 1g, 9g. The combination of 70g, 1 1g and 9g will do.