courses › Mental Arithmetic

The distance to 100 or 1,000, and moving the decimal point by tens.

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

Learn first (about 3 minutes)

Opens at level 2.

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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.

• Gap to 100 par 6 s not tried yet

  worked example

  100 − 99

  Answer: 1

  1. 99 → 100 is 1.

• Gap to 1,000 par 6 s not tried yet

  worked example

  1,000 − 655

  Answer: 345

  1. 655 → 700 is 45, and 700 → 1,000 is 300, so the gap is 345.

• × and ÷ by 10 and 100 par 6 s not tried yet

  worked example

  547 ÷ 100

  Answer: 5.47

  1. Dividing by 100 makes it 100 times smaller: every digit moves two places right, so 547 becomes 5.47.

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

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