The area of the original rectangle was wb.
The area of the altered rectangle is pw x 1.1b.
Since the two areas are equal,
1.1 pwb = wb
so
1.1 p = 1
so p = 1/1.1 = 10/11
So the width is 10/11 of its original value, so it has been decreased by 1/11, which as a percentage is 9 and 1/11 %, or 9.09 % to 2.d.p.