The difference between a million and a billion is hard to grasp, but a useful comparison is that a million seconds would last for about 11 days, whereas a billion seconds would last 31.4 years. If I had a pile of £1bn in £10 notes, how high would it be? I ask because replacing Trident is reputed to cost £205bn over its lifetime. Would that pile reach to the moon?
Caroline Westgate, Hexham, Northumberland
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