The whole area (shaded and un-shaded) is 5*10 units, or 50 units.
The shaded area is 3*6 units, or 18 units.
Subtract 50 - 18 to find the area of the un-shaded region → 32 units.
This means the probability that a point chosen in the large rectangle is un-shaded is 32/50 or 64%