Worked solution
Choose at least one of five funfair rides
There are 5 different rides in a fun fair. How many different combinations the child can make if the child has to take at least one ride?
CJC · Lecture · Example 17
Identify the source selection model
Choose an unordered subset of the five different rides. Each ride is either selected once or not selected. The source is not counting ordered visits or arbitrary repetitions.
Method 1: subtract the empty selection
For rides 1,2,3,4,5 respectively, the choices are take or do not take: 2 choices each. Therefore , subtracting the single case of taking no rides. The source caption beneath the fifth factor repeats “4th ride”; that factor belongs to the fifth ride.
Method 2: split by the number of selected rides
Taking 1,2,3,4,5 rides respectively gives , , , , . These cases are disjoint and exhaust all nonempty subsets, so the total is .
Common mistakes
“Combinations” means subsets here. Counting ride sequences or allowing unlimited repeated visits would be a different problem.