Step 1 (series): Total emf $=2E$, total resistance $=8+2r$. $1.5 = \dfrac{2E}{8+2r}$ ... (i)

Step 2 (parallel): Equivalent emf $=E$, equivalent internal resistance $=r/2$. $1.0=\dfrac{E}{8+r/2}$ ... (ii)

Step 3: From (i): $2E = 1.5(8+2r) = 12+3r \Rightarrow E=6+1.5r$.

Step 4: From (ii): $E = 1.0(8+r/2) = 8+0.5r$.

Step 5: Equate: $6+1.5r = 8+0.5r \Rightarrow r=2\Omega$.

Step 6: $E = 8+0.5(2) = 9$ V.