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Step 1:

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The emf of the battery is 25V.

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The internal resistance of the battery is .

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Two series circuits with branches of connected in parallel.

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Draw a circuit with above specifications :

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 Step 2:

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Find the equivalent resistance :

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Resistors are in series.

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Let is equivalent resistance of .

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Resistors are in series.

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Let is equivalent resistance of .

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Let is equivalent resistance of .

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Now is in series with the internal resistance of the battery.

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 Step 3:

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Find the current in the circuit :

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From ohms law : .

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Substitute in above formula.

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 Step 4:

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Apply Kirchhoffs current law (KCL) at each node :

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Apply Kirchhoffs voltage law (KVL) to first loop :

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Substitute in equation (1).

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Step 5 :

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The potential difference across the external resistor is .

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Substitute and in above equation.

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The potential difference across the external resistor is .

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Substitute and in above equation.

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The potential difference across the external resistor is .

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Substitute and in above equation.

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The potential difference across the external resistor is .

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Substitute and in above equation.

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The potential difference across resistor is .

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The potential difference across resistor is .

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The potential difference across resistor is .

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The potential difference across resistor is . 

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