77. Length: 51; width: 6

79. $x=4$

81. 1500

83.

1. 5 yr

2. $y=-{\displaystyle \frac{1}{18}}x(x-3)(x-8)$

3. export 1.7 million barrels

85.

1. $y={\displaystyle \frac{1}{54}}x(x-6)(x-8)$

2. $592.59 profit

3. 2004, 2007

4. $898.46 in 2006