87.

1. $C(x)=0.5x+2000$

2. $\overline{C}(x)=0.5+{\displaystyle \frac{2000}{x}}$

3. $\overline{C}(100)=20.5,$ $\overline{C}(500)=4.5,\overline{C}(1000)=\mathrm{2.5.}$ These show the average cost of producing 100, 500, and 1000 trinkets, respectively.

4. Horizontal asymptote: $y=0.5.$ It means the average cost (the fixed daily cost of producing each trinket) if the number of trinkets produced approaches $\infty$.

89.

1. 

2. 1. About 9 min

   2. About 12 min

   3. About 76 min

   4. 397 min

3. 1. $\infty$;

   2. Not applicable; the domain is $x<100$.

4. No

91.

1. $3.02 billion

2. 

3. 90.89%

93.

1. 16,000

2. The population will stabilize at 4000.

95.

1. $f(x)=(10x+200,000)/(x-2500)$

2. $40

3. More than 25,000

4. $x=2500(\text{free books});y=10(\text{cost per book})$