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In the following figure, lines PQ and RS intersect each other at point O. If ∠ POR : ∠ ROQ = 5 : 7, then the value of ∠ POS will be :

Line PQ crossing the vertical line RS at O, with S at the top and R at the bottom, P to the left and Q to the right

  • A.105°
  • B.75°
  • C.150°
  • D.210°

Answer: A

The value of ∠ POS is 105°.

∠ POR and ∠ ROQ lie on the straight line PQ, so they add up to 180°. Splitting 180° in the ratio 5 : 7:

\[ \begin{aligned} \angle POR &= \tfrac{5}{12} \times 180^\circ = 75^\circ \\ \angle ROQ &= \tfrac{7}{12} \times 180^\circ = 105^\circ \end{aligned} \]

∠ POS and ∠ ROQ are vertically opposite angles, so ∠ POS = 105°. 75° is ∠ POR (and its opposite ∠ SOQ), not ∠ POS; 150° and 210° double the parts.

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