There are 10 parking spaces numbered from 101 to 110. At least one car is parked in these slots. If cars can be parked only at the consecutively numbered parking slots, how many such arrangements can be made. Consider that only one car can be parked in one parking slot and all cars are identical.

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Case 1: 1 car in 10 slots: just pick any 1 slot: i.e. 10C1 = 10!/1!9! = 10, see combination formula here

Case 2: now lets say if 2 cars need to be parked in 10 slots, as the cars need to be parked in consecutive slots only, its similar to choosing 1 position out of 9 slots, i.e. 9C1 = 9

Case 2: now lets say if 2 cars need to be parked in 10 slots, as the cars need to be parked in consecutive slots only, its similar to choosing 1 position out of 9 slots, i.e. 9C1 = 9

if you go on like this, number of ways of parking 10 cars in 10 slots = 1

thus total possible arrangements = 10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 +1 = 10*11/2 = 55