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'Debasing the Coinage' printed from https://nrich.maths.org/
Answer: 240g
There are 100 5p's in £5.
The old coins weighed $100 \times 5.65 = 565$g, while the new coins weigh $100 \times 3.25 = 325$g.
So your pocket is $565-325=240$g lighter.