85. $[0,\infty );$ the particle’s motion is tracked indefinitely from time $t=0.$

87. The graph is above the t-axis on the intervals (0, 9) and (21, 24); the particle is moving forward between 0 and 9 sec and again between 21 and 24 sec.

89. The particle is moving with increasing speed forward on (0, 3), (5, 6), and (21, 23) and moving backward on (11, 15).

91. Maximum speed is attained between times $t=15$ and $t=16.$

93. The particle is moving forward with increasing velocity.

95.

2. (0, 6)

3. [0, 128]

4. When $x=2$, V has a maximum value of 128.

97.

1. $C(x)=210x+10\text{,}500$

2. $21,000

3. $420

4. 100

99. 350,000 students per year

101.

1. 4 ft from the origin

2. 28 ft

3. 6 ft/sec

4. 10 ft/sec