This combinations problem is trickier for young children than it appears, because most will not be systematic in creating different towers. Have plenty of blocks available so they can keep each tower as they make it. A good question to keep asking is `How do you know you have found all the towers?' 
 
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Encourage the children who cope well to go on and try it with four different colours, then five. The most able might be able to discover a pattern that will predict how many combinations will be found.
 
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