Method 1:

Take the bigger coin of radius R.. Draw a circle using this coin with the pencil.

Now take the small coin of radius 5mm and draw the circles using it along the circumference on the inside of the bigger circle approximately in one of the semicircles. keep filling it such that the smaller circles touch eachother but not overlap...

stop the above process untill it reaches the center...

u ve the radius from the count of the small circles along the diameter of the bigger circle.

Method 2:

if v r allowed to draw the diameter... it becomes more simple. just adjust the smaller coin along the diameter with the center on the diameter.

To draw the diameter v can simply draw the circle of the biggger coin.. and then fold the paper such that the semicircles.. coincide.

But.. there might be a shorter one??

Sorry i am bad at explaining thngs