For Joshy: Top Ten+ Arithme”tricks”

For Joshy (and the rest of us, too): I’m sure you’re working on your math tables, memorizing addition, subtraction, multiplication, and division of numbers from one through ten.
Do you need a shortcut for multiplying by four, five, nine, or eleven, or squaring two-digit numbers ending in five? How about subtracting a large number from 1,000? [...]