Multiple choice general knowledge math & puzzles

(1/10)18 - (1/10)20 = ?

  1. 0.9

  2. 99/10

  3. 99/10^20

  4. None of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Factor out (1/10)^18: (1/10)^18 × [1 - (1/10)^2] = (1/10)^18 × [1 - 1/100] = (1/10)^18 × 99/100 = 99/(10^18 × 10^2) = 99/10^20.

AI explanation

Compute (1/10)^18 - (1/10)^20 = 10^-18 - 10^-20. Factor out the smaller power: 10^-20 (10^2 - 1) = 10^-20 (100 - 1) = 99 × 10^-20, which can be written as 99/10^20. This is exact and matches option "99/10^20." The option "0.9" is far too large in magnitude (off by many orders of magnitude), and "99/10" is likewise off by 19 orders of magnitude — both are common mistakes from mishandling the exponents. "None of the above" is wrong since 99/10^20 is indeed listed and correct. 99/10^20 is the correct answer.

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