NCERT Solutions Ganita Prakash (Part 1) Chapter 4 Mind the Mistake, Mend the Mistake — Omission of the Multiplication Symbol
Book page 87 Updated on2026-09-05
Q1. If a = – 4, then 10 – a = 6.
Answer Mistake. The negative sign of a was ignored.
10 – a = 10 – (–4)
= 10 + 4
= 14
Why it happens: Subtracting a negative number is the same as adding its opposite. Whoever solved this simply wrote 10 – 4.
Q2. If d = 6, then 3d = 36.
Answer Mistake. 3d means 3 × d, not d × d.
3d = 3 × 6 = 18
Why it happens: 36 comes from 6 × 6. The number written in front of a letter-number is the multiplier, and here it is 3.
Q3. If s = 7, then 3s – 2 = 15.
Answer Mistake. The 2 was subtracted before multiplying.
3s – 2 = 3 × 7 – 2
= 21 – 2
= 19
Why it happens: 15 = 3 × (7 – 2). But 3s – 2 has two terms, 3s and –2. The multiplication inside the term 3s must be done first.
Q4. If r = 8, then 2r + 1 = 29.
Answer Mistake. The 2 and the 8 were written side by side as 28.
2r + 1 = 2 × 8 + 1
= 16 + 1
= 17
Why it happens: 2r is a product, not a two-digit number. Replacing r by 8 gives 2 × 8, never “28”.
Q5. If j = 5, then 2j = 10.
Answer No mistake. This one is correct.
2j = 2 × 5 = 10
Q6. If m = –6, then 3 (m + 1) = 19.
Answer Mistake. The bracket was not evaluated with the negative value.
3 (m + 1) = 3 × (–6 + 1)
= 3 × (–5)
= –15
Why it happens: –6 + 1 is –5, not 6 + 1. Substitute the whole value, sign included, before working inside the bracket.
Q7. If f = 3, g = 1 then 2f – 2g = 2.
Answer Mistake. The subtraction was done before the multiplications.
2f – 2g = 2 × 3 – 2 × 1
= 6 – 2
= 4
Why it happens: 2 comes from 2 × (3 – 1). Here 2f and 2g are separate terms; each must become a number first.
Q8. If t = 4, b = 3 then 2t + b = 24.
Answer Mistake. The + was treated as a ×.
2t + b = 2 × 4 + 3
= 8 + 3
= 11
Why it happens: 24 = 2 × 4 × 3. The expression has two terms, 2t and b, joined by addition.
Q9. If h = 5, n = 6 then h – (3 – n) = 4.
Answer Mistake. The bracket was handled wrongly.
h – (3 – n) = 5 – (3 – 6)
= 5 – (–3)
= 5 + 3
= 8
Why it happens: 3 – 6 is –3, and subtracting –3 means adding 3. You can also open the bracket first: h – 3 + n = 5 – 3 + 6 = 8. Both routes agree.