Ant climb up 5 stairs, each of length 20 cm and height 20 cm . Find the distance covered and displacement covered by ant ?

by "length" do you mean the tread width? Otherwise, what difference does it make how long the step is?

Anyway, just figure the distance to cover one step (up and across), and multiply by 5.

then use the Pythagorean Theorem on the total horizontal and vertical distance traveled.