SSC CGL 2025 Tier-I · 15 September 2025, Shift 2 · Quantitative Aptitude · Question 17 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 11 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.

If the perimeter of a regular octagon is 80 cm, what is the side length?

(a)10 cm
(b)8 cm
(c)12 cm
(d)6 cm

Answer

Why

Correct — A.
A regular octagon has 8 equal sides.
Perimeter = 8 × side
Side = 80 ÷ 8 = 10 cm → option (a)

Why the others are wrong

(b)
8 cm — 8 cm borrows the number of sides as the length. Eight sides of 8 cm give a perimeter of 64 cm, not 80 cm.
(c)
12 cm — 12 cm per side gives 8 × 12 = 96 cm, which is 16 cm more than the stated perimeter.
(d)
6 cm — 6 cm per side gives 8 × 6 = 48 cm, well short of the 80 cm perimeter.

Concept

A polygon is regular when all its sides are equal and all its angles are equal. Its perimeter is then simply number of sides × side length.

An octagon has 8 sides, so each side is one-eighth of the perimeter. The same division works for any regular polygon once you know how many sides its name stands for.

Key facts

Octagon = 8 sides, hexagon = 6, pentagon = 5, decagon = 10.
Perimeter of a regular n-sided polygon = n × side.
Each interior angle of a regular octagon = (8 − 2) × 180° ÷ 8 = 135°.

Study next

Interior and exterior angles of a regular polygonArea of a regular hexagon from its side, as six equilateral trianglesNumber of diagonals of an n-sided polygon: n(n − 3) ÷ 2

Common traps

Mixing up the octagon (8 sides) with the decagon (10 sides): 80 ÷ 10 gives 8 cm

18 Sep 2025, 12:30, Quant Q.18 asks for each interior angle of a regular eight-sided polygon: (8 − 2) × 180° ÷ 8 = 135°.

17 Sep 2025, 16:00, Quant Q.18 asks for the total of the interior angles of a ten-sided polygon: (10 − 2) × 180° = 1440°.

Related PYQs

No directly related past PYQ was found.

Topic: Triangle, Quadrilaterals, Regular Polygons, Circle — all 65 SSC CGL questions on it, every solved shift.