Core skills · Level 8

Direct & inverse proportion

More means more, or more means less: unitary method problems.

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The idea

Direct: more of one, more of the other. Inverse: more of one, less of the other.

Look: Direct goes up in a straight line. Inverse comes down as x grows.

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✗More workers take more days.
✓More workers finish sooner: inverse.
✗Thinking everything is proportional.
✓Your height does not double when your age doubles!

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Direct: more of one, more of the other. Inverse: more of one, less of the other.

Direct: find the value for one, then multiply. Inverse: the total stays the same, so share it.

Think of it like thisMore pens cost more money (direct). More workers finish the job in fewer days (inverse).

Direct goes up in a straight line. Inverse comes down as x grows.

Question

12 workers build a wall in 15 days. How long would 18 workers take?

More workers, fewer days: inverse. The total work stays the same.
12 × 15 = 180 worker-days
Share the 180 among 18 workers.
180 ÷ 18 = 10

Answer10 days.

Kanakku makes new questions on direct & inverse proportion every time. It shows the working and gives a hint when you are stuck.