2014 AMC 10B Problem 7

By definition, we know $$A = \dfrac {x+ 100}{100}B$$$$= B + \dfrac x{100}B.$$ This implies, $$\begin{aligned} (A-B) &= \dfrac x{100}B\\ 100\left(\dfrac{A-B} B\right) &=x.\end{aligned}$$ Thus, the correct answer is A .