[填空题]
The fourth term, $u_4$ , of a geometric sequence is 135. The sixth term, $u_5$ , is 81.
Find the common ratio of the sequence.
r= .
Find $u_1$ , the first term of the sequence.
$u_1$= .
Calculate the sum of the first 20 terms of the sequence.
$S_{20}$= .
