I've never seen this trick before, but it appears to work...   (I had to try it out an see for myself, though.)
Once you figure this out it'll blow your mind...

"The lines over the circles are color coded. Notice the single red line and 3 blue lines representing "13" grouped together while the single green and 2 black lines take their own group. [Simply] draw your first group of lines in one direction then your second group of lines going over the first, count the groups of intersections and there's your answer."


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