85. (80, 30)

87. (17, 4)

89. The diameter of the largest pizza is 21 inches, and the diameter of the smallest pizza is 8 inches.

91. 8% of the total trash collected is plastic, and 40% is paper.

93. 340 beads and 430 doubloons

95. 3 Egg McMuffins and 2 Breakfast Burritos

97. $18,000 at 7.5% and $32,000 at 12%

99. $7.50 per hour at McDougal’s and $15 per hour for tutoring

101. 20 pounds of herb and 80 pounds of tea

103. The speed of the plane is 550 km/hr, and the wind speed is 50 km/hr.

105.

1. $C(x)=30\text{,}000+2x;R(x)=3.5x$

2. 

3. 20,000 magazines

107. Her weekly sales should be more than $6250.

109. (3, 9)

111. (ln 4, ln 2)

113. $c=2$

115. $c={\displaystyle \frac{-14}{5}}$

119. $y={\displaystyle \frac{-7}{2}}x+{\displaystyle \frac{13}{2}}$

121.

1. $x={\displaystyle \frac{b_2{c}_1-b_1{c}_2}{a_1{b}_2-a_2{b}_1}},y={\displaystyle \frac{a_1{c}_2-a_2{c}_1}{a_1{b}_2-a_2{b}_1}},$ where ${\text{a}}_1{b}_2-a_2{b}_1\ne 0$

2. ${\displaystyle \frac{c_1}{b_1}}\ne {\displaystyle \frac{c_2}{b_2}},{\displaystyle \frac{a_1}{a_2}}={\displaystyle \frac{b_1}{b_2}}$

3. ${\displaystyle \frac{a_1}{a_2}}={\displaystyle \frac{b_1}{b_2}}={\displaystyle \frac{c_1}{c_2}}$

123.

1. $b(x-x_1)-a(y-y_1)=0$

2. $\left({\displaystyle \frac{b^2{x}_1-ab{y}_1-ac}{a^2+b^2}},{\displaystyle \frac{-ab{x}_1+a^2{y}_1-bc}{a^2+b^2}}\right)$

125. (6, 7)