Question

Year .. Invention
----------------------------
1807.. Steamboat
1825 ..Photography
1837.. Telegraph
1846.. Sewing machine
1869.. Hydroelectric power
1876.. Telephone
1879.. Light bulb
1888 ..Rabies vaccine
1890 ..Diesel engine
1897 ..Airplane
--------------------------------


In Mr. Tarkiainen’s history class, each student chooses two of the topics listed in the chart above for their papers. How many different pairs can Juliana choose?

10
45
30
90

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.