Children in the sixth grade should have learned that "two triangles with the same height have the same area", right?

Solution: Connect CI and FC.

Because there are three squares in the figure, FD and CI are parallel.

So triangle DFC and triangle DFI are two triangles with the same base and the same height.

And the DFC area of triangle =10 * (10-8)/2 =10.

So the DFI area of the triangle is 10.