A rectangle has a fixed area of 84 square metres. Its length is changed from 12 metres to 21 metres. What must its new width be? Enter the number of metres.

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The area remains length times width, so the original width is 84 divided by 12, or 7 metres, while the new width is 84 divided by 21, or 4 metres. Scaling the width upward with the length would enlarge the area instead of keeping 84 square metres fixed.