201902180817 Electric circuit diagrams (Elementary) Q1

The blogger claims no originality of his question below.

0001

In the figure above, resistors R_1, R_2, R_3 and R_4 are connected in series. It is known that R_3=R_4. After the circuit is connected to a battery, the voltmeter readings of V_1 and V_2 are 10\,\mathrm{V} and 5\,\mathrm{V} respectively. Find the potential difference (p.d.) across points a and b.


Answer. 15\,\mathrm{V} is an answer.