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The best CAPTCHA ever - 2

I already wrote about The best CAPTCHA ever — it’s a simple, but impressive looking math expression. Use of a scientific CAPTCHA is taking off. Now you can see one on the registration page of the library of Moscow Institute of Physics and Technology:

circuit captcha


In order to launch the registration process, please pass a quick test. It has only one school-level problem.

You need to determine the resistance between the points A and B in the circuit.