What kind of sequence is this? 39, 156, 468, 936

Ican figure out 20 shares multiply by $117.98 =2359.6920 shares multiply by $124.32=2486.40then take 40 shares multiply by $128.48=5139.20after that i'm lost any be greatly appreciated.

The sales tax rate is 7.25%. how much tax in dollars is added on an item that costs $56.00? a. $4.06 b. $7.25 c. $14.50 d. $60.06

Find the tenth term in the following geometric sequence. 8, 4, 2, 1, . .a) 13b) 0.0078c) 0.0156d) 12.5