Add checked answer key and calculations for all 20 questions

DynamoDB/Practice/Read and Write Units - Set 3.md

@@ -94,3 +94,35 @@ Answer: ______
 ### 20. You perform eventually consistent reads of five separate 12.01 KB items every second. What is the minimum WHOLE NUMBER of RCUs you must provision?
 
 Answer: ______
+
+## Answer key
+
+<details>
+<summary>Show answers and calculations</summary>
+
+`ceil` means round up to the next whole number. Sizes are divided by the 1 KB write or 4 KB read increment before rounding.
+
+1. **12 WCUs** — `ceil(1.8 / 1) × 6 = 2 × 6`.
+2. **9 WCUs** — `ceil(0.7 / 1) × 9 = 1 × 9`.
+3. **16 WCUs** — `ceil(3.01 / 1) × 4 = 4 × 4`.
+4. **18 WCUs** — `ceil(2.01 / 1) × 3 × 2 = 3 × 3 × 2`.
+5. **12 WCUs** — `ceil(1 / 1) × 6 × 2 = 1 × 6 × 2`.
+6. **10 RCUs** — `ceil(5 / 4) × 5 = 2 × 5`.
+7. **9 RCUs** — `ceil(12 / 4) × 3 = 3 × 3`.
+8. **4.5 RCUs** — `ceil(0.8 / 4) × 9 × 0.5 = 1 × 9 × 0.5`.
+9. **7.5 RCUs** — `ceil(8.01 / 4) × 5 × 0.5 = 3 × 5 × 0.5`.
+10. **7 RCUs** — `ceil(8 / 4) × 7 × 0.5 = 2 × 7 × 0.5`.
+11. **20 RCUs** — `ceil(4.01 / 4) × 5 × 2 = 2 × 5 × 2`.
+12. **18 RCUs** — `ceil(11 / 4) × 3 × 2 = 3 × 3 × 2`.
+13. **11 WCUs** — `(ceil(0.6 / 1) × 2) + (ceil(2.4 / 1) × 3) = 2 + 9`.
+14. **5.5 RCUs** — `(ceil(5 / 4) × 2) + (ceil(1 / 4) × 3 × 0.5) = 4 + 1.5`.
+15. **7 writes/s** — `28 / ceil(3.2 / 1) = 28 / 4`.
+16. **5 transactional item writes/s** — `30 / (ceil(2.2 / 1) × 2) = 30 / 6`.
+17. **10 reads/s** — `15 / (ceil(9 / 4) × 0.5) = 15 / 1.5`.
+18. **8 transactional item reads/s** — `32 / (ceil(5 / 4) × 2) = 32 / 4`.
+19. **6 provisioned RCUs** — `ceil(ceil(3 / 4) × 11 × 0.5) = ceil(5.5)`.
+20. **10 provisioned RCUs** — `ceil(ceil(12.01 / 4) × 5 × 0.5) = ceil(10)`.
+
+For questions 1–14, RCUs describe exact throughput requirements. Provisioned settings must be whole numbers, as explicitly requested in questions 19–20.
+
+</details>