two poles of height 25 cm & 20 cm are are parallel to each other with a distance of 12 cm between them.
what is the distance between the top of the two poles?
draw a right triangle with the tops of the poles as the ends of the hypotenuse, and the bottom side perpendicular to the poles.
You have a 5-12-13 triangle.
It is useful to learn some of the basic triangles with integer sides -- they pop up frequently.
3-4-5
5-12-13
8-15-17
7-24-25
and multiples of these