How do you quickly multiply two numbers both between 10 and 20, e.g. 13 × 14?

Add the whole first number to the second's units digit, times ten, plus the product of the units.

For (10+a)(10+b): (first number + b)·10 + a·b:

• 13 × 14 → (13 + 4)·10 + 3·4 = 170 + 12 = 182.
• 17 × 16 → (17 + 6)·10 + 7·6 = 230 + 42 = 272.

Check: 13 × 14 = 182. ✓ Because (10+a)(10+b) = 100 + 10(a+b) + ab = 10·(10 + a + b) + ab.