Ans : 16
Name the 10 people with letters: A, B, C, ... and so on. A through F each call any one of G, H, I, or J (it doesn't matter which one). That makes six calls so far. Then G calls H and I calls J, after which G calls I and H calls J. Now we've used 10 calls, and G, H, I, and J all know everything. Finally, each of A through F is called by someone in G through J - 6 more calls to get everyone knowing everything. 16 calls in all.
Name the 10 people with letters: A, B, C, ... and so on. A through F each call any one of G, H, I, or J (it doesn't matter which one). That makes six calls so far. Then G calls H and I calls J, after which G calls I and H calls J. Now we've used 10 calls, and G, H, I, and J all know everything. Finally, each of A through F is called by someone in G through J - 6 more calls to get everyone knowing everything. 16 calls in all.
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