If we start with 10x then 10% less ie 90% of the original value we get
10x(1-0.1) = 9x
We then put 10% back on, but now it will be 10% of the 9x so we won't get 10x but rather:
9x(1+0.1) = 9.9x.
In one step it would be
10x(1-0.1)(1+0.1) which is the difference of two squares and simplifies to 10(1-0.01) = 9.9