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In a room, there are three-legged stools and four-legged chairs. When people sat down on all of these seats, there were 39 legs (human and stool/chair legs) in the room. How many stools are there in the room?

One day a strange notebook was found on the stairs. It contained one hundred statements:

“There is exactly one incorrect statement in this notebook”;

“There are exactly two incorrect statements in this notebook”;

“There are exactly three incorrect statements in this notebook”;

...

“There are exactly one hundred incorrect statements in this notebook.”

Are any of these statements true, and if so, which ones?

Jack the goldminer extracted 9 kg of golden sand. Will he be able to measure 2 kg of sand in three goes with the help of scales: a) with two weights of 200 g and 50 g; b) with one weight of 200 g?