Answer:
x=1000 or 10 to the 3rd power
7/21 or 1/3rd, it's just a ratio of white to all, there's 7 white straws and 21 in all, then you can simplify it down to 1/3rd.
#15(A)
First Bike
$380 - $190 ($380 * 0.50) = $190
2nd Bike
$380 - $114 ( $380 * 0.30) = $266
$266 - $53.20 ($266 * 0.20) = $212.80
#15 (B)
Eliza should buy the first bike because it is cheaper. $190 < $212.80
#16
$57 + $28.50 ($57 * .50) = $85.50