There are 10 stations on a railway line. The number of different journey tickets that are required by the authorities is
The number of different journey tickets that are required by the authorities = The number of selecting 2 stations $$\times\ $$ 2 (For every pair stations say A and B, there are two tickets required A to B and B to A)
= $$\\^{10}C_2$$ * 2 = 45*2=90
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