Post Utme Babcock University 2018 Physics Question 6

A parallel plate capacitor with plates of area $A$ and separation $d$ is connected to a battery of emf $V$. If the dielectric cons\tant of the material between the plates is $kappa$, the capaci\tance of the capacitor is given by $C = \frac{kappa A epsilon_0}{d}$. If the battery is disconnected and a dielectric slab of thickness $t$ and dielectric cons\tant $kappa'$ is inserted between the plates, the new capaci\tance is given by $C' = \frac{kappa' A epsilon_0}{d - t}$. What is the ratio of the new capaci\tance to the original capaci\tance?