NCERT Solutions Ganita Prakash (Part 1) Chapter 3 In-text Questions — Units of Measurement

Book page 65 Updated on2026-09-05

Q1.

Fill in the blanks below (mm <–> cm): 12 mm = 1.2 cm; 56 mm = 5.6 cm; 70 mm = _______; ________ = 0.9 cm; 134 mm = __________; ________ = 203.6 cm

Answer

Given	Working	Answer
12 mm	12 ÷ 10	1.2 cm (given)
56 mm	56 ÷ 10	5.6 cm (given)
70 mm	70 ÷ 10	7.0 cm
? = 0.9 cm	0.9 × 10	9 mm
134 mm	134 ÷ 10	13.4 cm
? = 203.6 cm	203.6 × 10	2036 mm

Why it happens: 1 mm = ¹⁄₁₀ cm. So mm → cm means divide by 10 (decimal point one step left) and cm → mm means multiply by 10 (decimal point one step right).

Tip: 7.0 cm and 7 cm are the same length — the zero after the point adds nothing, but it does remind you that the measurement was made to the nearest millimetre.

Q2.

The illustration below shows how small some things are! Try taking an approximate measurement of each.

Answer

These are all smaller than one millimetre or just about one millimetre, so they must be described using tenths and hundredths of a millimetre.

Object	Size as printed	Same size in cm
Fine pen stroke	0.5 mm	0.05 cm
Medium pen stroke	1 mm	0.1 cm
Bold pen stroke	1.5 mm	0.15 cm
Human hair (thickness)	about 0.1 mm	0.01 cm
Newspaper (thickness)	0.05 to 0.08 mm	0.005 to 0.008 cm
Mustard seed (thickness)	1 – 2 mm	0.1 – 0.2 cm
Smallest ant, Carabera Bruni	0.8 – 1 mm	0.08 – 0.1 cm
Smallest land snail, Acmella Nana (shell)	0.7 mm	0.07 cm

Why it happens: a ruler's smallest mark is 1 mm, so none of these can be measured directly. We say a hair is about 0.1 mm because roughly 10 hairs side by side span one millimetre — the tenth again, one level smaller.

Try This: stack 20 sheets of a newspaper, measure the stack with a ruler, and divide by 20. If the stack is 1.4 mm, one sheet is 1.4 ÷ 20 = 0.07 mm.

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