20. List the natural numbers from $1,2,3, \cdots, 99,100$ that are both odd and composite in a row, such that no two adjacent numbers are coprime (if one line is not enough, continue on the second line, and if the second line is still not enough, continue on the third line).
【Answer】Solution: Natural numbers that are both odd and composite with a common divisor of 3: $9, 15, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99$;

With a common divisor of $5: 25, 35, 55, 65, 85, 95$;
With a common divisor of 7: 49, 77, 91;
Arrange the numbers so that no two adjacent numbers are coprime: $25, 35, 55, 65, 85, 95, 15, 9, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99, 77, 91, 49$.

Therefore, the answer is: $25, 35, 55, 65, 85, 95, 15, 9, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99, 77, 91, 49$.