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Find from Factors

Age 11 to 14 Short
Challenge Level

A certain number has exactly eight factors including $1$ and itself. Two of its factors are $21$ and $35$. What is the number?
 
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
  
This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.
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