NCERT Solutions Ganita Prakash (Part 1) Chapter 4 In-text Questions — The Notion of Letter-Numbers
Book page 81 Updated on2026-09-05
Q1.
Example 1: Shabnam is 3 years older than Aftab. When Aftab’s age 10 years, Shabnam’s age will be 13 years. Now Aftab’s age is 18 years, what will Shabnam’s age be? _______
Answer
Shabnam is always 3 years older, so add 3 to Aftab's age.
Shabnam's age = Aftab's age + 3 = 18 + 3 = 21 years
Why it happens: The gap between two people's ages never changes. Aftab was 10 when Shabnam was 13 — a gap of 3. That same gap of 3 stays for ever, so we can add 3 to any age of Aftab.
Q2.
Given Aftab’s age, how will you find out Shabnam’s age?
Answer
Add 3 to Aftab's age.
Shabnam's age = Aftab's age + 3
Aftab's age
Shabnam's age
4
4 + 3 = 7
10
10 + 3 = 13
23
23 + 3 = 26
Tip: Describe the rule in plain words first. The words tell you exactly what to write in symbols.
Q3.
Can we write this as an expression?
Answer
Yes. Give Aftab's age a short name and the rule becomes one line.
Let a = Aftab's age, s = Shabnam's age Shabnam's age = Aftab's age + 3 s = a + 3
Why it happens: Writing ‘Aftab’s age’ again and again is slow. A single letter a stands for that number, whatever it is. Letters used this way are called letter-numbers, and an expression like a + 3 is an algebraic expression.
Q4.
If a is 23 (Aftab’s age in years), then what is Shabnam’s age?
Answer
Replace a by 23 in the expression a + 3.
s = a + 3 = 23 + 3 = 26 years
Check it yourself: Put a = 40. Then s = 43 — and 43 – 40 = 3, the same gap. The formula never breaks.