Method - 1:

Required percentage = (100 * 20)/(100 + 20)

                                    = (2000)/(120)

                                    = 16.666667

                                    = 16 2/3%.

Method - 2:

Let the height of B be 'x'.

Given that A's height is 20% more than B.

⇒ x + 20% of x

⇒ x + (20/100) * x

⇒ x + (20x/100)

⇒ (120x)/100

⇒ 6x/5

Their Difference:

⇒ (6x/5) - x

⇒ (x/5).

Now,

Required percentage =  (x/5) * (5/6x) * 100%

⇒ (1/6) * 100%

⇒ 50/3 %

⇒ 16 2/3%.

Hope it helps!