A basketball player made baskets during a game. Each basket was worth either or points. How many different numbers could represent the total points scored by the player?
- A)
- B)
- C)
- D)
- E)
Answer
E
Key insight
Each time a 2-point basket becomes a 3-point basket, the total goes up by exactly 1. Six choices for the number of 3-pointers give six totals.
Solution
The only thing that changes the total is how many of the baskets are -pointers. So list every possibility.
- The number of -pointers can be or . The rest are -pointers.
- Work out each total:
| -pointers | -pointers | Total |
|---|---|---|
- The six totals are all different.
So there are possible totals.
The answer is .
Why this works
Trading one -point basket for a -point basket adds exactly point. So the totals climb by one at a time, from (all twos) to (all threes), and no total repeats. When a question asks "how many different totals", pick one count to vary and list the cases; remember to include both ends, zero and all.
The trap
Answering 5 because there are five baskets, and forgetting the cases where all baskets are 2-pointers or all are 3-pointers.
Concise solution
Let be the number of -point baskets, so the other baskets are worth points each. The total is
Since can be or , the possible totals are : six different values, each achieved by exactly one .
The answer is .
Review in the AoPS books
- Introduction to Counting & Probability, Chapter 1: Counting Is Arithmetic
- Introduction to Algebra, Chapter 3: One-Variable Linear Equations (§3.3, §3.4)
- Introduction to Counting & Probability, Chapter 2: Basic Counting Techniques
- Introduction to Algebra, Chapter 7: Proportion (§7.4)
Techniques
Set up the equation/formula and compute; no special trick needed