Practical circuits — practice question

As shown, two batteries are joined together. Battery 1 has electromotive force (e.m.f.) $12\,\text{V}$ and internal resistance $0.3\,\Omega$. Battery 2 has e.m.f. $9\,\text{V}$ and internal resistance $0.1\,\Omega$. Determine the e.m.f. and the internal resistance of one battery that would produce the same effect as this arrangement.