courses › Thinking

Tricks that make hard-looking products quick.

Best in your head · no pen, no calculator why?

Learn first (about 3 minutes)

Opens at level 19.

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Builds on: Bigger multiplication (not open yet)

skills · practice stats

From rounds of this course only: box, review and trial answers are left out. Once a skill has 40 tries, it compares your first 20 tries with your last 20.

• × 11 par 6 s not tried yet

  worked example

  50 × 11

  Answer: 550

  1. Put the digit sum between the digits: 5 + 0 = 5.
  2. So 5, 5, 0: 550.

• Squares ending in 5 par 6 s not tried yet

  worked example

  75²

  Answer: 5,625

  1. Take the tens part times one more: 7 × 8 = 56.
  2. Then write 25 after it: 5,625.

• Around a round number par 8 s not tried yet

  worked example

  23 × 17

  Answer: 391

  1. Both are 3 away from 20: 20² − 3² = 400 − 9 = 391.

• Near 100 par 10 s not tried yet

  worked example

  99 × 95

  Answer: 9,405

  1. Both are near 100, short by 1 and 5.
  2. Cross-subtract: 99 − 5 = 94, and 94 hundreds is 9,400.
  3. Multiply the shortfalls: 1 × 5 = 5. 9,400 + 5 = 9,405.

rest ladder

Win 3 of your last 4 rounds and the course rests. A win is 90% right, within 2× the round's par. Pass the review when it comes back and the next rest is longer.

1. 1 day
2. 3 days
3. 7 days
4. 14 days
5. 30 days
6. 60 days
7. mastered · every 90 days

your rounds

No rounds yet.