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NCERT Solutions Ganita Prakash (Part 1) Chapter 4 –86Section 4.2 Revisiting Arithmetic Expressions — In-text Questions

Book page 85 Updated on2026-09-05

Q1.
Let us revise these concepts and find the values of the following expressions: 1. 23 – 10 × 2 2. 83 + 28 – 13 + 32 3. 34 – 14 + 20 4. 42 + 15 – (8 – 7) 5. 68 – (18 + 13) 6. 7 × 4 + 9 × 6 7. 20 + 8 × (16 – 6)
Answer

Write each expression as a sum of terms, turn each term into a number, then add.

1. 23 – 10 × 2 = 23 + (–10 × 2) = 23 + (–20) = 3
2. 83 + 28 – 13 + 32 = (83 – 13) + (28 + 32) = 70 + 60 = 130
3. 34 – 14 + 20 = (34 – 14) + 20 = 20 + 20 = 40
4. 42 + 15 – (8 – 7) = 57 – 1 = 56
5. 68 – (18 + 13) = 68 – 31 = 37
6. 7 × 4 + 9 × 6 = 28 + 54 = 82
7. 20 + 8 × (16 – 6) = 20 + 8 × 10 = 20 + 80 = 100
Why it happens: In (2) we used swapping and grouping — 83 and –13 go together, 28 and 32 go together. In (5) the two ways shown in the book agree: 68 – (18 + 13) = 68 – 31 = 37, and also 68 – 18 – 13 = 50 – 13 = 37. A minus sign outside a bracket flips the sign of every term inside.
Tip: A term that contains × must be turned into a single number before you add. That is why 23 – 10 × 2 is 3 and not 26.
Q2.
Find an algebraic expression to get the nth term of this sequence: 4, 8, 12, 16, 20, 24, 28, ...
Answer

These are the multiples of 4 in increasing order, so the term at position n is 4 times n.

3rd term = 4 × 3 = 12
29th term = 4 × 29 = 116
nth term = 4 × n = 4n
Why it happens: The 1st term is 4 × 1, the 2nd is 4 × 2, the 3rd is 4 × 3 … The position number is always the multiplier, so position n gives 4 × n. As a standard practice we drop the × sign and write 4n, number first, letter after.
Q3.
Find the value of the expression 7k when k = 4. Find the value that the expression 5m + 3 takes when m = 2.
Answer

Remember 7k means 7 × k and 5m means 5 × m.

7k when k = 4 → 7 × 4 = 28
5m + 3 when m = 2 → 5 × 2 + 3 = 10 + 3 = 13
Tip: 5m + 3 is not 5 × (m + 3). Multiply first, then add — 5m is one term and 3 is another.
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