One of the possible solutions is shown in the upper illustration.
If we mark all straight cuts from the center of the cake to its edges, as
shown in the lower illustration, we get exactly 20 triangles. All the
triangles are equal in area - they have equal bases and the same altitude.
Therefore to get five equal parts of the cake we have to divide it into five pieces consisting of 4 small triangles each (20:5=4) as shown in the lower illustration.