Arithmetic

• How to Multiply, Divide, Add, and Subtract Fractions
  Learn the rules for multiplying, dividing, adding, and subtracting fractions, with worked examples, simplification tips, and common mistakes to avoid.
• Factorial, but with Addition
  Replacing the products in n! with addition gives 1 + 2 + … + n, the triangular number n(n + 1)/2; here is why.
• How Much Did the Shop Lose in the Stolen-Money Riddle?
  In the classic stolen-money riddle, the shop loses $100 at selling price: $70 in goods and $30 in change, not $200.