courses › Mental Arithmetic

Add and subtract two- and three-digit numbers in one pass.

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

Learn first (about 3 minutes)

Opens at level 4.

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Builds on: Small facts

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.

• Two-digit addition par 6 s not tried yet

  worked example

  79 + 18

  Answer: 97

  1. Round and compensate: 79 + 20 = 99, then take back the extra 2: 97.

• Two-digit subtraction par 9 s not tried yet

  worked example

  44 − 28

  Answer: 16

  1. Count up from 28: 2 gets to 30, then 14 more gets to 44.
  2. 2 + 14 = 16.

• Three-digit compensation par 9 s not tried yet

  worked example

  449 − 382

  Answer: 67

  1. Subtract a round number, then fix it: 449 − 380 = 69.
  2. That took away 2 too few, so take away 2 more: 67.

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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