SSC CGL 2025 Tier-I · 20 September 2025, Shift 1 · Quantitative Aptitude · Question 19 of 100

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 13 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

When a transversal intersects two parallel lines, one of the alternate interior angles is 115°. What is the measure of the angle that lies opposite to it on the alternate interior side?

(a)65°
(b)90°
(c)180°
(d)115°

Answer

Why

Correct — D. With parallel lines, alternate interior angles are equal.

Given angle = 115°, one of an alternate interior pair
Its partner sits on the other parallel line, across the transversal
Partner = given angle = 115° → option (d)

Why the others are wrong

(a)
65° — 65° is 180° − 115°, the co-interior angle on the same side of the transversal. Co-interior angles add to 180°, while the alternate angle asked for equals 115°.
(b)
90° — 90° needs the transversal to be perpendicular to both lines, which makes every angle 90°. An angle of 115° shows it is not perpendicular.
(c)
180° — 180° is the sum of two co-interior angles, 115° + 65°, not the measure of one angle. Each angle where two lines cross is less than 180°.

Concept

A transversal crossing two parallel lines makes eight angles, four at each crossing. They fall into pairs:

Alternate interior: between the lines, on opposite sides of the transversal. Equal.
Corresponding: in matching positions at the two crossings. Equal.
Co-interior: between the lines, on the same side. They add to 180°.

Every one of the eight angles is either x or 180° − x, here 115° or 65°.

The stem's wording, the angle that lies opposite to it on the alternate interior side, is loose.

Read as the other angle of the alternate interior pair, it is 115°. Read as the vertically opposite angle at the same crossing, it is also 115°. Both readings reach the key.

Key facts

With parallel lines, alternate interior angles are equal.
Co-interior (same-side interior) angles add up to 180°.
Vertically opposite angles are equal at any crossing, parallel lines or not.
Adjacent angles on a straight line add up to 180°, so 115° sits beside 65°.

Study next

Corresponding and co-interior angle pairs with parallel linesAngle chasing through a bend between two parallel linesProving two lines parallel from one equal pair of angles

Common traps

Answering 65°: that is the co-interior angle, which is supplementary to 115°, not equal to it.
Mixing up alternate angles (opposite sides of the transversal) with co-interior angles (same side).

The stem names the angle pair in words and gives no figure, so it tests whether you know which pair is equal and which pair adds to 180°.

Related PYQs

No directly related past PYQ was found.