Since Q is the midpoint of line segment PR, the value of x is equal to 3 units.
In Mathematics, the midpoint of a line segment with two end points can be calculated by adding each end point on a line segment together and then divide by two (2).
Since Q is the midpoint of line segment PR, we can logically deduce the following:
Line segment PQ = Line segment QR
PR = PQ + QR
By substituting the given line segments into the equation above, we have the following:
20 = 2x + 4 + 2x + 4
20 - 8 = 4x
12 = 4x
x = 12/4
x = 3 units.