courses › Thinking

Get a feel for thousands, millions, and billions.

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

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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.

• Big-number names par 6 s not tried yet

  worked example

  A billion times 100 is:

  1. a hundred million
  2. a hundred billion
  3. a hundred trillion
  4. a trillion

  Answer: a hundred billion

  1. Count zeros: a billion has 9 and 100 has 2. 9 + 2 = 11 zeros: a hundred billion.

• Daily to yearly par 13 s not tried yet

  worked example

  A dog walker costs $135 a week. How much is that in a year of 52 weeks?

  Answer: $7,020.00

  1. $135 × 52 = $135 × 50 + $135 × 2 = $6,750 + $270 = $7,020.

• How long is that? par 10 s not tried yet

  worked example

  About how long is 2 million hours?

  1. 230,000 years
  2. 2,300 years
  3. 23,000 years
  4. 230 years

  Answer: 230 years

  1. A year has 8,760 hours (365 × 24).
  2. 2,000,000 ÷ 8,760 ≈ 228, so about 230 years.

• Powers of ten par 14 s not tried yet

  worked example

  Write 4 as a × 10ⁿ with 1 ≤ a < 10. What is n? (Use a minus sign if n is negative.)

  Answer: 0

  1. 4 is already between 1 and 10, so the point doesn't move.
  2. So 4 = 4 × 10⁰, and n = 0.

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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