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# Column subtraction 4: Lesson 2: brittle stars

## Learning Objective

We are learning how to subtract a 3-digit or a 4-digit number from a 4-digit number using column subtraction (with more than one exchange, including exchanges across more than one place value).

Context: Breathtaking Brittle Stars

https://www.bbc.co.uk/programmes/p094lpfx/player

Credit: BBC Two - Winterwatch

## Clip Description

Brittle stars are related to starfish. They are so named because of their brittle arms, which can detach from their body if disturbed. Common brittle stars are found all over the UK in rockpools and offshore to depths of 85 metres. They are often found in dense clusters of up to 2,000 individuals per square metre.  They come in many different colours, from dull brown to purple, red, orange and yellow. Their five spiny legs are patterned with coloured bands.

In this fascinating clip, thousands of brittle stars can be seen feeding on plankton on the floor of Loch Carron. With the arrival of each new moon, the brittle stars must work as a team to stop being washed away by powerful ocean currents.

## Whiteboard Number Challenge

We have learnt that brittle stars are often found in dense clusters of up to 2,000 individuals per square metre.

Imagine that your classroom floor was covered with brittle stars!

With 2,000 per square metre, how many brittle stars would cover the floor of your classroom?