Answer :
To estimate the total cost, I would say that it is roughly $2 per gallon, and she is buying roughly 9 gallons, so the total cost ≈ 2 x 9 = $18.
9.16 * 1.99
= 9.16 * 1
+ 9.16 * 0.99 = around 9.07
9.16 + 9.07 = 18.23
$18.23
= 9.16 * 1
+ 9.16 * 0.99 = around 9.07
9.16 + 9.07 = 18.23
$18.23