Number of ends in a line  = $0$ = $X$

Number of ends in a line segment = $2$ = $Y$

$Y-X=2-0=2$

You know that a rectangle is a parallelogram. In the figures below, we can see that we can make rectangles and also a parallelograms. Also we can make a pentagon. But making a rhombus is impossible by using $2$ set squares having ${30}^{\circ }$- ${60}^{\circ }$ - ${90}^{\circ }$ angles.

If you draw any of the diameters and join the ends you will always get a rectangle as shown below in figure 1.

Figure 1

Figure 2

It is possible to construct an angle equal to ${7.5}^{\circ }$ using compass and ruler.
Steps to draw are:

·Draw an angle of ${60}^{\circ }$ using compass and ruler.

·Bisect it to get ${30}^{\circ }$.

·Bisect it to get ${15}^{\circ }$.

·Bisect it to get ${7.5}^{\circ }$.
So we can construct an angle of ${7.5}^{\circ }$.

The four tools used for drawing shapes are:
i) Rulers
ii) Compasses
iii) Set squares 
iv) Protractor

Construction: 1 Mark
Diameter: 1 Mark
The circle can be drawn using a ruler and compass.

The diameter of the circle
 $=2\times$ radius $=2\times 10=20cm$

Construction of Angles: 1 Mark each
                     ${60}^{\circ }$ angle

               ${30}^{\circ }$ angle

Constructing PQ: 1 Mark
Steps: 2 Mark
Steps for constructing PQ based on the length of AB: 
(i) Let AB be the line segment.

(ii) Adjust the compass up to the length of AB.

(iii) Draw any line l and mark a point P on it.

(iv) Put the pointer on P and without changing the setting of the compass, draw an arc to cut the line segment at point X.
(v) Now, put the pointer on X and again draw an arc with the same radius as before, to cut the line l at the point Q.

PQ is the required line segment.

Construction: 1 Mark
Solution: 1 Mark
The figure shows a line segment AB.

From a single point, an infinite number of lines can be drawn.
$\therefore$ From point A, infinite lines can be drawn. 

Each steps: 1 Mark
Step 1: Draw a line l. Mark a point A on the line.

 
Step 2: Place the compass pointer on the zero mark of the ruler. Open it to place the pencil point up to the 5.6 cm mark.
Step 3: Taking caution that the opening of the compass has not changed the measured length, place the pointer on A and swing an arc to cut l at B.
Step 4: AB is a line segment of the required length.
​