NCERT Solutions for Class 9th Maths Chapter 7 Exercise Set 7.1 — The Probability Scale
Book page 159 Updated on2026-09-08
Q1.
Rank the following events on a scale from 0 (Impossible) to 1 (Certain). Label each event: Impossible, less likely, equally likely (even chance), more likely, certain. Give reasons why you gave each event its ranking. (i) The next Monday will come after Sunday. (ii) It will snow in Mumbai in July. (iii) An elephant will walk through your classroom today. (iv) You will greet at least one friend at school tomorrow.
Answer
Event
Label
Where it sits
Reason
(i) The next Monday will come after Sunday
Certain
P = 1
Not a random event at all. The order of the days is fixed by the calendar, so there is no other possible outcome.
(ii) It will snow in Mumbai in July
Impossible
P = 0
Snow needs the air to be at or below 0 °C. Mumbai is coastal and tropical; July is its monsoon, with temperatures around 25–30 °C. The favourable outcome does not exist.
(iii) An elephant will walk through your classroom today
Impossible (or as near 0 as makes no difference)
P ≈ 0
Classroom doors are not elephant-sized and elephants are not loose in towns. In a forest-edge school with a wild-elephant corridor nearby it would be "extremely unlikely" rather than strictly impossible.
(iv) You will greet at least one friend at school tomorrow
More likely
P close to 1
You have many friends and you greet them every day, so it is far more likely to happen than not. It is not certain, though — tomorrow could be a holiday, or you could be absent.
The four events placed on the probability scale of Fig. 7.1.
Why it happens: the scale is doing the same job a number line does for length. Every event has to land somewhere between 0 and 1, and the further right it sits, the more confident we are. What decides the position is evidence, not opinion: (i) is settled by definition, (ii) by climate records, (iii) by what a classroom physically is, (iv) by your own daily experience.
Tip: keep "impossible" for events with no favourable outcome — P is exactly 0 — and use "extremely unlikely" for events that are merely rare, like (iii) at a forest-edge school. A very small probability is still not zero, and the difference matters once you start calculating.