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A see-saw plank rests on a pivot. On one side the plank makes an angle of 24° with the level ground. What angle does the plank make with the ground on the other side of the pivot?

Class 7Lines and AnglesLines and Angles
  • A24°
  • B66°
  • C336°
  • D156°

Step-by-Step Solution

The ground is a straight line, so the two angles the plank makes with it form a linear pair and sum to 180°. Other angle = 180 − 24 = 156°. Trap: 66° is 90 − 24, the complement — that would be right only if the ground turned through a right angle at the pivot, which it does not.
Answer: D) 156°

Key idea: Vertically opposite angles

When two straight lines cross, four angles form at the intersection. The two angles directly **across** the vertex (sharing only the vertex, not a side) are called **vertically opposite** — and they are always equal.

If one angle is 70°, then:
- Its vertical opposite is also 70°.
- The other two angles are each 110° (linear pair with 70°).

Why? Each pair forms a linear pair with a common neighbour, both summing to 180° — so the opposites end up equal.

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