In fact, because of the overlap, only 150.

Overlap =270- 150= 120

This 120 includes 3 overlapping areas +2 overlapping areas.

Actual: 2 * (overlapping area of 3 sheets)+1* (overlapping area of 2 sheets) = 120.

That is, the center blank area *2+ shadow area = 120.

Shadow area = 120-2 * 28 = 64cm2.