The brainbuster

Find the solution to this brainbuster and win £200 of Amazon vouchers to spend on high-tech!

It's time for the ACME Ltd 'Spring into Summer' staff event. Amongst the refreshments, there's a 10 litre punchbowl filled to the brim with ACME's legendary punch. Now, a jug (capacity J litres) is filled from the bowl, and the punchbowl topped up with water. Assume there's no spillage, and the punchbowl's contents mix completely.

Then the process is repeated once more - the jug is filled with liquid from the punchbowl, and the bowl topped up with water.

Unfortunately for latecomers to the event, the proportion of punch to water in the punchbowl is now 1:1.

What is the capacity of the jug?

Show your workings. Only complete entry forms will be considered. The judge's decision is final.

The competition closes on Sunday 22nd July 2018.