DIY simple cardboard life hacks

In a class, teacher gave two cardboard pieces having equal area which are in the shape of a parallelogram to two groups. Find h.

A source S and a detector D are placed at a distance d apart. A big cardboard is placed at a distance sqrt(2) d from the source and the detector as shown in figure. The source emits a wave of wavelength = d//2 which is received by the detector after reflection from the cardboard. It is found to be in phase with the direct wave received from the source. By what minimum distance should the cardboard be shifted away so that the reflected wave becomes out of phase with the direct wave?

If the thickness of a pile of 12 cardboards is 35 mm, find the thickness of a pile of 294 cardboards.

If the thickness of a pile of 16 cardboards is 44 mm.how many cardboards will be there in a pile which is 71.5 cm thick ?

The area of a rectangular piece of cardboard is 36 sq cm and its length is 9cm. What is the width of the cardboard?

If the thickness of a pile of 12 cardboards is 65 mm, find the thickness of a pile of 312 such cardboards.

Draw a cage on one side of a cardboard and a bird on the other side. Hang the cardboard with the help of a thread. Twist the thread and leave it. What do you see and why ?

Letters A, B, C and D are written on a cardboard as shown in the figure below. The cardboard is kept at a suitable distance behind a transparent empty glass of cylindrical shape. If the glass is now filled with water, one sees an inverted image of the pattern on the cardboard when looking through the glass. Ignoring magnification effects, the image would appear as