During the Beijing Olympics, a unit in Beijing made a promotional placard as shown in the picture below. The circumference of the placard is 16 meters.

As shown in the picture, the promotional placard is a "parallelogram". Suppose the longer side of the parallelogram is x, then the shorter side is [(16/2)-x];

According to the principle of constant area of ??parallelogram: S=3*x=5*[(16/2)-x], from 3*x=5*[(16/2)-x], we can get x =5, so the area of ??the quadrilateral is:

S=3*5=15.