Multiple choice general knowledge math & puzzles

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

  1. 0.9

  2. 99/1020

  3. 99/10

  4. none of these

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Assuming the notation means (1/10)^18 - (1/10)^20: Factor out (1/10)^18 to get (1/10)^18[1 - (1/10)^2] = (1/10)^18[1 - 1/100] = (1/10)^18(99/100) = 99/10^20 = 99/1020. This uses exponent factoring.