Question

Year .. Invention

Answer 1

The answer is 90.

Since there are 10 topics listed in the chart above, and if you pair the first topic Steamboat you will get 9 pairs by pairing to all of the topics. And the same result you will get if you pair the last topic Airplane.

So if there are 10 topics and each will get 9 pairings , so the over all pairs you will get is 90.