Praxis Computer Science (5652)

1. Question 1 · Impacts of Computing

   A teacher wants students at two different schools to co-write the same program. Students must be able to work at the same time, see who made each change, and return to an earlier working version if a change breaks the program.

   Which approach best meets all three needs?
   • A.A video call where one student types while the others watch and comment
   • B.Emailing a zipped copy of the project to the other school every evening
   • C.A shared repository with version control that logs each change by author
   • D.A single cloud folder where each new upload replaces the previous file
   Show answer and explanation

   Answer: C. A shared repository with version control that logs each change by author

   Version control lets many contributors work in parallel, records the author of every commit, and allows reverting to any earlier version, so it meets all three needs. Emailing zipped copies prevents simultaneous work and creates conflicting copies; a cloud folder that overwrites files loses the history needed to roll back; a video call allows only one typist and keeps no change record.

2. Question 2 · Algorithms and Computational Thinking

   Which binary value is equal to the hexadecimal number 2B9?
   • A.0010 1011 1001
   • B.0010 1101 1001
   • C.0010 1100 1001
   • D.1001 1011 0010
   Show answer and explanation

   Answer: A. 0010 1011 1001

   Each hex digit maps to exactly four bits: 2 = 0010, B (11) = 1011, 9 = 1001. Writing B's bits backward gives 1101 (which is D), reversing the digit order gives 1001 1011 0010, and treating B as 12 gives 1100.

3. Question 3 · Programming

   Pseudocodeint r ← 7 + 12 % 5 * 2 print r

   Operators *, /, and % have equal precedence, are evaluated left to right, and are evaluated before + and -. What is printed when the code segment is executed?
   • A.8
   • B.9
   • C.11
   • D.19
   Show answer and explanation

   Answer: C. 11

   12 % 5 is 2, then 2 * 2 is 4, and 7 + 4 is 11. "8" comes from adding first ((7 + 12) % 5 * 2 = 4 * 2); "9" comes from multiplying 5 * 2 before taking the remainder (12 % 10 = 2, then 7 + 2).

4. Question 4 · Programming

   Pseudocode// discount applies to orders of // 10 through 49 items, inclusive boolean hasDiscount(int qty) return qty ≥ 10 and qty ≤ 49 end hasDiscount

   A tester can run only four test cases. Which set of qty values best checks the boundaries of the discount rule?
   • A.0, 25, 100, 1000
   • B.9, 10, 49, 50
   • C.9, 11, 48, 50
   • D.10, 20, 30, 40
   Show answer and explanation

   Answer: B. 9, 10, 49, 50

   Boundary-value testing uses the values on each edge and just outside it: 9 and 10 at the lower limit, 49 and 50 at the upper limit. These would expose a mistake such as > instead of ≥. "9, 11, 48, 50" looks close but skips the exact boundaries 10 and 49, and the other sets test only typical or extreme values.

5. Question 5 · Data

   A simulation models a cafeteria line using randomly generated student arrival times and a fixed serving time of 20 seconds per student. Which question can this model answer most appropriately?
   • A.Which particular student will arrive first tomorrow
   • B.How the average wait changes if a second serving station opens
   • C.Whether students will like the new lunch menu better
   • D.Exactly how long the 50th student will wait next Monday
   Show answer and explanation

   Answer: B. How the average wait changes if a second serving station opens

   A simulation built from arrival patterns and serving times is well suited to comparing scenarios, such as one versus two stations, by averaging many runs. Because arrivals are random, it cannot predict exact waits or the identity of a particular student on a real day. Food preferences are not part of the model at all, so the model has no data to answer that question.

6. Question 6 · Impacts of Computing

   A science class uploads weekly bird counts to an online database that combines reports from thousands of volunteers. Researchers use the combined data to detect changes in migration timing.

   Which statement best describes the role of computing in this project?
   • A.It aggregates scattered contributions into data no single group could gather
   • B.It guarantees each entry is accurate, because digital records cannot be wrong
   • C.It replaces human observers, since the database itself detects the birds
   • D.It mainly protects each volunteer's privacy by encrypting all of the counts
   Show answer and explanation

   Answer: A. It aggregates scattered contributions into data no single group could gather

   This is crowdsourcing: networked computing lets many distributed observers contribute to one dataset at a scale no single class could reach, enabling new research. People still make the observations, digital entries can contain human errors that researchers must clean, and the project's purpose is collaboration and discovery, not encryption.

7. Question 7 · Algorithms and Computational Thinking

   The decimal number 237 is converted to 8-bit binary and then to hexadecimal. What is the hexadecimal result?
   • A.DE
   • B.ED
   • C.EF
   • D.CD
   Show answer and explanation

   Answer: B. ED

   237 = 128 + 64 + 32 + 8 + 4 + 1 = 1110 1101 in binary. The left group 1110 is E and the right group 1101 is D, giving ED (check: 14 × 16 + 13 = 237). "DE" reverses the digit order, and "EF" and "CD" come from mistakes in the binary step.

8. Question 8 · Programming

   Pseudocodeint a ← 9 int b ← 4 double c ← 2.0 print a / b * c print a * c / b

   In this language, / between two int values performs integer division, / with a double operand performs floating-point division, and * and / are evaluated left to right. What are the two printed values?
   • A.2.0 and 4.5
   • B.4.0 and 4.5
   • C.4.0 and 4.0
   • D.4.5 and 4.5
   Show answer and explanation

   Answer: B. 4.0 and 4.5

   In the first line, 9 / 4 is integer division (2), and 2 * 2.0 is 4.0. In the second line, 9 * 2.0 is the double 18.0, and 18.0 / 4 is 4.5. "4.5 and 4.5" ignores integer division, and "2.0 and 4.5" stops after the division.

9. Question 9 · Programming

   Pseudocode// returns the average of a double avg(int[] a) int total ← 0 for (int i ← 1; i < a.length; i ← i + 1) total ← total + a[i] end for return total / a.length end avg

   In this language, / divides as floating-point division even when both values are int. The procedure is called as avg({4, 8, 6}). Which describes the error in the procedure and the value returned?
   • A.A logic error; it returns about 4.67
   • B.A syntax error; the loop header is invalid
   • C.A logic error; it returns exactly 6.0
   • D.A runtime error; the index goes out of bounds
   Show answer and explanation

   Answer: A. A logic error; it returns about 4.67

   The loop starts at index 1, so a[0] = 4 is skipped and total = 8 + 6 = 14. The procedure runs without crashing but returns 14 / 3 ≈ 4.67 instead of 6.0, which is a logic error. The index never exceeds a.length − 1, so there is no runtime error, and "returns exactly 6.0" is the intended result, not the actual one.

10. Question 10 · Data

    Pseudocodeint hits ← 0 for (int i ← 0; i < 100; i ← i + 1) int r ← randomInt(1, 6) if (r ≥ 5) hits ← hits + 1 end if end for print hits / 100

    randomInt(1, 6) returns a random integer from 1 to 6, inclusive. Which change would most likely make the printed estimate closer to the true probability on every run?
    • A.Change both 100s to 100000
    • B.Change r ≥ 5 to r > 5
    • C.Change randomInt(1, 6) to 5
    • D.Change hits / 100 to hits / 6
    Show answer and explanation

    Answer: A. Change both 100s to 100000

    The code estimates the chance of rolling 5 or 6, which is 2/6 ≈ 0.33. With more trials, random variation averages out, so the estimate lands closer to 0.33 more consistently. Changing the condition to r > 5 estimates a different event (about 0.17), dividing by 6 is not a proportion of trials, and replacing the roll with a constant removes the randomness the model depends on.

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