It's been a while

Hello there. I've been writing, honestly. My next novel is so near to being completed I can taste it, and then perhaps I'll rejoin the real world for a while. But in the meantime, I stumbled across a puzzle I wrote while I was working on Perplex City. I think it was deemed unsuitable to make it onto a puzzle card, so I present it to you here in all its glory. Enjoy.

Hmm, I guess if I'm presenting a puzzle I should probably also offer a prize. Well... how about the first person to email me the correct answer to this puzzle gets... a bag of pretzels. And I can throw a signed copy of Disobedience in there too.

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Child Care

It's a shame how few people read small print in contracts. If Andrea Phillips, our Ad-hoc Polymath had taken greater care examining her contract of employment, she'd have noticed the clause under which her first-born child is indentured to work at Mind Candy for seven years. Andrea may not be happy, but Mind Candy is concerned about the most efficient way to gain access to their new employee.

Since Sasha is in New York and the Mind Candy office is in London, she will have to be posted to her employers. Mind Candy is not heartless, however, and is willing to provide for food for the journey to be packaged with Sasha.

Sasha can subsist on a diet of hard and soft pretzels and apple juice for up to three months. Each hard pretzel weighs 5 grams, each soft pretzel weighs 150 grams. Sasha needs 750 grams of pretzels and 1 litre of apple juice a day. Sasha herself weighs 15 kilos. Every day of Sasha's work is worth £10 to Mind Candy. Of course, time spent in transit has no value.

Air freight parcels will be delivered in 10 days, while surface parcels will take 3 weeks. If the USPS charges £9 for the first pound and £2.40 for each subsequent pound for air freight, and charges £13 for the first 5 pounds and £0.80 for each subsequent pound for surface mail, is it more efficient for Sasha to be air-freighted from New York to London packaged in pretzels, or to be sent by surface mail? What is the difference, to the nearest £10?