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Question P6 Repeated identity

Today’s problem — free all day, every step. Every day has one.

A box of cube candles — large and small

A candle maker packs a rectangular storage box tightly with 27 identical large cube-shaped candles and some identical small cube-shaped candles, so that candles of the same size are stacked directly on top of each other, filling the box exactly to the brim. The figure shows the first layer of candles packed in the box.

  1. (a) How many small candles does he have?
  2. (b) The volume of the box is 37800 cm³. The total volume of the 27 large candles is of the volume of the box. What is the length of one edge of the small candle?
The first layer of the box seen from above: 9 large candles on the left and 10 small candles on the right, both blocks running the full depth of the box. Drawn to scale.

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About this method

Repeated identity

One length or count, described two ways in the same question.

In this one. The figure gives no lengths — but two blocks spanning the same depth fix the edge ratio 5 : 3, and that ratio alone counts every candle; the volume is only needed at the very end.

the same edge, measured twicelong tilesshort tilesboth rows end level, so 3 long = 5 short

How to spot it

A shape or count is described from two viewpoints in the same question, such as one edge measured as a number of long pieces and again as a number of short pieces.

Common mistake

Children treat the two descriptions as separate quantities and add them. Both describe the same quantity, so the two must be equal.

Practise a similar question

Standard A rectangular tank measures 30 cm by 20 cm by 25 cm. It is 3/5 full of water. All the water is poured into a second rectangular container with a base area of 200 cm². What is the height of the water in the second contain…
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