courses › Mental Arithmetic

Single-digit addition and subtraction until the answer is instant.

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

Learn first (about 3 minutes)

New. Learn first, or start a round.

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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.

• Sums to 18 par 3 s not tried yet

  worked example

  8 + 7

  Answer: 15

  1. Near double: 7 + 7 = 14, and one more is 15.

• Differences under 20 par 3 s not tried yet

  worked example

  12 − 4

  Answer: 8

  1. Back to ten first: 12 − 2 = 10, then 2 more back is 8.
  2. Check: 4 + 8 = 12.

• Missing addend par 3 s not tried yet

  worked example

  7 + ? = 15

  Answer: 8

  1. Count up through ten: 7 → 10 is 3, 10 → 15 is 5, and 3 + 5 = 8.

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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