“Arr!” said the Mathematical Pirate.
“Pieces of eight!” said the Mathematical Pirate’s parrot.
“How many pieces of eight?”
“That’ll be… seventy and ten minus twenty and four, making fifty and six!”
“Who’s a clever boy?” asked the parrot. “Awk!”
The Mathematical Pirate is, indeed, a clever boy. He’s using a combination of number bonds and the small times tables he knows by h-arrrr-t.
The way it works is this:
Alternatively, he could have worked the same thing the other way: since $2 + 8 = 10$, $8\times$ something is $10\times$ the thing minus $2\times$ the thing. Seven tens are 70, which he thinks of as sixty-ten; seven twos are 14; taking the ten from the 60 leaves 50, and taking the 4 from the 10 leave 6. 56 again!
This is a really powerful trick — the Mathematical Pirate claims he learnt it from a Ninja, but nobody saw it.
In a more general form, if you need to work out (big times table number) × (something else) — for instance, $7\times4$ you’ll need: