You've Unlocked a New GMAT® Question!

Solution:

Let’s follow the steps we learned above to determine y:

Step 1: List the given equations.

1) 5x = y

2) z = 2y

3) x + 9 = z

Step 2: Combine any two equations.

We can combine equation 1 and equation 2. Since 5x = y, we can substitute 5x for y in equation 2, and we have the following:

$\begin{array}{l}\Rightarrow z=2y\\ \Rightarrow z=2\left(5x\right)\\ \Rightarrow z=10x\end{array}$

Step 3: Combine the simplified equation with the remaining untouched equation, and then solve for an unknown variable.

We can combine the new equation above with equation 3. Since z = 10x, we can substitute 10x for z in equation 3, and we have the following:

$\begin{array}{l}\Rightarrow x+9=z\\ \Rightarrow x+9=10x\end{array}$

Now, since we have one equation with just one variable (x), we can determine the value of that variable (x).

$\begin{array}{l}\Rightarrow x+9=10x\\ \Rightarrow 9=9x\\ \Rightarrow 1=x\end{array}$

Step 4: If required, plug the value found into the equations in order to find the values of the other variables.

We can substitute 1 for x in equation 1 in order to determine y:

$\begin{array}{l}\Rightarrow 5x=y\\ \Rightarrow 5\left(1\right)=y\\ \Rightarrow 5=y\end{array}$