A 10 V battery with internal resistance 1 $$\Omega$$ and a 15 V battery with internal resistance 0.6 $$\Omega$$ are connected in parallel to a voltmeter (see figure). The reading in the voltmeter will be close to:

The formula for the equivalent EMF of two cells connected in parallel (with the same polarity) is:

$$E_{eq} = \frac{E_1 r_2 + E_2 r_1}{r_1 + r_2}$$

$$E_{eq} = \frac{\frac{E_1}{r_1} + \frac{E_2}{r_2}}{\frac{1}{r_1} + \frac{1}{r_2}}$$

$$E_1 r_2 = 10 \times 0.6 = 6$$, $$E_2 r_1 = 15 \times 1 = 15$$, $$r_1 + r_2 = 1 + 0.6 = 1.6$$

$$E_{eq} = \frac{6 + 15}{1.6}$$

$$E_{eq} = \frac{21}{1.6}$$

$$E_{eq} = 13.125 \text{ V}$$