Family fortune

If three brothers will get £20 more if they do not share their money with their sister, how much money is there?
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Problem

Three brothers and a sister shared a sum of money equally among themselves. If the brothers alone had shared the money, then they would have increased the amount they each received by £20. What was the original sum of money?

 

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.