Two capacitors, each having capacitance 40 are connected in series....

When a dielectric is inserted between the sheets of a capacitor, its capacitance becomes $KC$.

Now the equivalent capacitance in series combination will be,

${C}_{eq} = \dfrac{C \times KC}{C\left(1 + K\right)} ⇒ 24 = \dfrac{40K}{1 + K} ⇒ K = 1 . 5$