Equivalent systems of equations

The  exercise appears under the Algebra I Math Mission. This exercise practices finding equivalent systems of equations.

Types of problems
There is one type of problem in this exercise:
 * 1) Which of them obtained a system that is equivalent to the teacher's system? This problem has a story of a teacher giving two students a linear equation to solve, and user is asked to find which student(s) have a system that is equivalent to the teacher's.

Strategies
Basic knowledge of equations are needed for this exercise.
 * 1) Systems of equations that have the same solution are called equivalent systems.
 * 2) Given a system of two equations, user can produce an equivalent system by replacing one equation by the sum of the two equations, or by replacing an equation by a multiple of itself.
 * 3) In contrast, user can be sure that two systems of equations are not equivalent if they know that a solution of the one is not a solution of the other.