64seconds% N3 n; A6 h! ?+ W
: J! O2 u2 O7 x7 k: j5 r* C1 to 4 have 3floors = 48 sec* q* V% \# X2 {8 \: U; A
avg. 48/3 = 16sec
! Q% X8 @4 `' W( t& t' r, vtvb now,tvbnow,bttvb& V6 r3 D+ G5 ]$ Z, r/ Q2 S, {0 o
4 to 8 have 4floors = 16x4) R) g- w5 S$ C1 o. ~6 [
= 64secs |