100 + 5% = 105

105 - 5% = 11.75 why ? And how to get 100

5% of 100 added to 100 is indeed 105.

But 5% 0f 105 subtracted from 105 is

105-.05(105)=99.75, not 11.75.

I think you are asking, "what percentage of 105 subtracted from 105 gives 100?".

105-105x=100.

Solve for x. It is not 5.

You add 5% to 100 and get to 105. But 5% of 105 is a different number. You're then figuring 5% of 105, not of 100 anymore, so it's a different answer. That's "how come."