Let the number be 100.
When 10% is increased = 100 x (110/100) = 110

Now, to keep area unchaged we should have = [(110 -100)/110] x 100
= (10/110) x 100
= 100/11
= 9(1/11)%  (mixed fraction of 100/11)
Therefore, the height of the triangle is to be decreased by 9(1/11)%.