courses › Mental Arithmetic

Add lists smartly and estimate before you calculate.

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

Learn first (about 3 minutes)

Opens at level 11.

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Builds on: Two-digit add & subtract (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.

• Group into hundreds par 9 s not tried yet

  worked example

  850 + 530 + 450

  Answer: 1,830

  1. Pair the friendly numbers first: 850 + 450 = 1,300, then add 530: 1,830.

• Estimate by rounding par 9 s not tried yet

  worked example

  Estimate 8,130 ÷ 91 using 8,100 ÷ 90.

  Answer: 90

  1. Use the friendly pair: 8,100 ÷ 90 = 810 ÷ 9 = 90.

• Right size? par 6 s not tried yet

  worked example

  Which is closest to 50 × 5,636?

  1. about 2,800,000
  2. about 28,000,000
  3. about 280,000
  4. about 28,000

  Answer: about 280,000

  1. Round each number to its leading digit: 50 × 6,000 = 300,000.
  2. The exact 281,800 is about 280,000; the other choices are about 10 times too small, or 10 or 100 times too big.

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.