Q4. Convert into basic SI units (NCERT Exercise 1.27, page 27): (i) 28.7 pm; (ii) 15.15 pm; (iii) 25 365 mg.
1 pm = 10 m and 1 mg = 10 kg. (i) 28.7 × 10 m = 2.87 × 10 m. (ii) 15.15 × 10 m = 1.515 × 10 m. (iii) 25 365 × 10 kg = 2.5365 × 10 kg.
Q5. How many significant figures should the answer to 5 × 5.364 have (NCERT Exercise 1.31(ii), page 28)?
Four: 5 × 5.364 = 26.82. Read as a count, 5 is an exact number with infinite significant figures (NCERT page 13), so the answer keeps the four of 5.364; give four in an exam. Strictly, if the 5 were a measured value with one significant figure, the product could be reported only as 3 × 101 (reasoned).
Q6. Find 25.37 − 4.8 to the correct number of figures.
25.37 − 4.8 = 20.57. 4.8 has one decimal place, so the answer keeps one: 20.6.
Q7. Round 1.346 to two significant figures, and explain why rounding first to 1.35 gives a wrong answer.
The first digit dropped is 4, which is less than 5, so the answer is 1.3. Rounding in two steps (1.346 → 1.35 → 1.4) adds a second rounding to the first and pushes the result up by mistake: 1.346 is nearer 1.3 than 1.4.