What number should replace the question mark? The three hexagons, with numbers read as left-hand column (top to bottom) and right-hand column (top to bottom), are: first 2, 3, 6 and 4, 7, 2; second 1, 9, 7 and 3, 9, 4; third 4, 1, 8 and 8, 3, ?. Answer with a natural number.
Two columns of numbers start from 1 and are written row by row: the first column is 1, 3, 7, 15, 31 and the second column is 1, 2, 5, 26, ?. What number should replace the question mark? Answer with a natural number.
Four meshing cogs have 17, 8, 5 and 4 teeth. How many revolutions must the largest cog (17 teeth) make to bring all the cogs back to their original positions? Answer with a natural number.