Worked Example 1
Find the circumference of a circle of radius 14 cm. Take pi as 22/7.
Step 1: check what you were givenThe question says radius, so use C = 2 pi r rather than pi d.
Step 2: substituteC = 2 × 22/7 × 14. The 7 divides into the 14 exactly twice.
Step 3: finishC = 2 × 22 × 2 = 88 cm.
Sense checkThe diameter is 28 cm, and 3 times 28 is 84. Our 88 is a little more than that, so it is believable.
Worked Example 2
A bicycle wheel is 70 cm across. How many turns does it make over 1 kilometre?
Step 1: one turnOne turn covers C = 22/7 × 70 = 220 cm.
Step 2: match the units1 km = 100000 cm. Both numbers must be in centimetres before you divide.
Step 3: divide100000 ÷ 220 = 454.5, so about 455 turns.
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