The challenge is to make 20 on your calculator using only six keys.
4  7    2  7  =  20 
5  +  7  +  8  =  20 
1  2  0  ÷  6  =  20 
20 
How many different ways can you do this? You could try making a different total. Is there a limit to the possible totals? What if you could use seven keys or five keys?
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30 − 10 = 20 
10 + 10 = 20 
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