TASK DEFINITION: You are given a mathematical question described with an open-ended vocabulary. Questions in this task involve real-world situations, describing a mathematical problem. You are also given 4 or 5 answer options (associated with "A", "B", "C", "D", "E"). Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D', 'E'. LaTeX mathematical format (the standard way to express mathematical expressions in the typesetting software known as LaTeX) is used to express equations. Each question is solvable with high school math knowledge. Give only one answer for each question.
PROBLEM: The total monthly cost c to send \(x\) text messages is given by the function \(C(x) = 0.05x - 2.5\) for all \(x \geq 51\). If Hannah received a bill for \$10.50 for sending text messages last month, how many text messages did she send?
(A)60 (B)125 (C)160 (D)225 (E)260

SOLUTION: E

PROBLEM: After 21 students were added to a class, there were four times as many students as before. How many students were in the class before the addition?
(A)3 (B)7 (C)11 (D)14 (E)17

SOLUTION: B

PROBLEM: If \(p\) gallons of 10 percent antifreeze solution is added to \(q\) gallons of 20 percent antifreeze solution, what is the percent antifreeze of the resulting solution in terms of \(p\) and \(q\)?
(A)\(\frac { p + q } { 2 } \) (B)\(\frac { 10p + 20q } { 30 } \) (C)\(\frac { 10p + 20q } { p + q } \) (D)\(\frac { 10p + 20q } { 100 } \) (E)\(\frac { 100p + 200q } { p + q } \)

SOLUTION:
C