The correct answer is 50%.
Approximately the middle 50% of data is contained inside the box of a box plot.
The perimeter for this rectangle is: 276
The first quartile would be 48.5
You can always add or subtract
same value to/from both sides of equation. The same is true about multiplication and division.
In your case you have to subtract
x from both sides of equation in order to get rid of it on the left side:


But it's easier to remember that if you want to move some member of equation to other side, you just have to
change it's sign. Let's practice a bit using your equation (I have added "plus" signs where they are usually omitted to better understend what's going on):

Move
2y to the right side:

Move
8 to the left side:

Move both
x and
2y to the right side: