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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.

How did this part go?

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 (2×2×2×2×2)−1=25−1=31(2\times2\times2\times2\times2)-1=2^5-1=31, 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.

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Method 2: split by the number of selected rides

Taking 1,2,3,4,5 rides respectively gives (51)=5\binom51=5, (52)=10\binom52=10, (53)=10\binom53=10, (54)=5\binom54=5, (55)=1\binom55=1. These cases are disjoint and exhaust all nonempty subsets, so the total is 5+10+10+5+1=315+10+10+5+1=31.

How did this part go?

Common mistakes

“Combinations” means subsets here. Counting ride sequences or allowing unlimited repeated visits would be a different problem.

Read the hint