If order doesn't matter, there are 10 ways to chose the first ride, 9 ways to choose the second, and 8 ways to choose the third. This product (720) has you riding the same 3 coasters in 6 different orders. Dividing by 6 gives ...
720/6 = 120 different combinations of roller coasters
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This number might be written as 10C3 (choose 3 from 10). The meaning of that notation is ...