How Effective is Captcha?

Captcha is a method to reduce spam on the internet. Spambots crawl thew internet looking for ways to post links. If they can register for a website, they can often post links. One type of captcha makes users select pictures of a certain type. The captcha below contains nine pictures.

The users is expected to select the three pictures containing signs with street names. There are  
\[{}^9 C_3 = \frac{9!}{3! (9-3)!} ==84\]
  ways of doing this, so the probability of randomly picking the 3 correct pictures is  
This means that a spambot will guess wrong  
\[\frac{83}{84} \times 100 \% - 98.8 \%\]
  of the time.

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