MathBrush
←  All problems

Question P6 Repeated identity

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

A crate of chocolate cubes — large and small

A chocolate factory packs a rectangular crate tightly with 18 identical large chocolate cubes and some identical small chocolate cubes, so that cubes of the same size are stacked directly on top of each other, filling the crate exactly to the brim. The figure shows the first layer of cubes packed in the crate.

  1. (a) How many small chocolate cubes are there?
  2. (b) The volume of the crate is 5625 cm³. The total volume of the 18 large cubes is of the volume of the crate. What is the length of one edge of the small cube?
The first layer of the box seen from above: 6 large chocolate cubes on the left and 25 small cubes on the right, both blocks running the full depth of the box. Drawn to scale.

See all 10 steps

MathBrush is being tested with a few parent groups, and every solution on the site is open to them — one household, all children. If you were given a code, it goes in once and this device remembers it.

I have a code → See a free one like it

65 worked examples are free without a code, two from every pattern. No account, either way.

About this method

Repeated identity

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

In this one. The figure alone fixes L : s = 5 : 3 — one edge written in terms of the other — so the crate's height can be counted in layers of either cube long before any volume is known.

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…
All 16 with this pattern →
Get the free patterns and skills booklet

Patterns and skills booklet

Worked examples of all 33 patterns and skills, in a booklet you can print.