A circle has two measurements worth knowing: the distance all the way round it (the circumference) and the space inside it (the area). Both are built from the radius: circumference = 2 × π × r, which is the same as π × d, and area = π × r × r. π is not a tidy number, so the paper always tells you which value to use — take π = 22/7 when the radius or diameter is a multiple of 7, and π = 3.14 otherwise. Read the question twice: half the marks lost here are lost by using the diameter where the formula wants the radius.
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