Kempus,
I think the only way of learning mental arithmetic is to do it. I don't know of any books. That doesn't mean there aren't any, but if there are, I don't know how useful they'd be.
Next time you're in the supermarket, add up the contents of your trolley before you get to the checkout, to the nearest pound. When you're on the motorway and you pass a sign telling you how far you have to go, work out how far you've come already, what speed you're doing, and how long it will take, at that speed, to get to where you're going. Check the TAFs every day and work out the cross-wind component on the runway at your local airfield, even if you're not flying. Basically, practice practice practice. The best thing is, it's free!
FFF
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