<a id="ch-10"></a>

## Chapter 10 — Two minutes is 13:46 to 13:48

<!-- open-cyber-academy:lesson-figures:start ch-10 -->
### Lesson map

Lesson 10. Map, example, EX056.

```mermaid
flowchart TD
  A["13:46 advert"] --> B["13:48 file"]
  B --> G["two minutes, same afternoon"]
  R["14:20 report"] --> D["14:25 Wi-Fi off"]
  D --> P["14:30 permission"]
  G --> Q{"Is about 13:50 an exact minute?"}
  Q -->|No| K["Do not subtract it"]
```

Some pairs subtract cleanly. The remembered phrase does not.

### Logical drawing

```text
exact, Tuesday
  13:46 to 13:48 = 2 minutes
  14:20 to 14:25 = 5 minutes
  14:25 to 14:30 = 5 minutes
  14:20 to 14:30 = 10 minutes
not exact
  about 13:50
other day
  Wednesday 15:10
```
<!-- open-cyber-academy:lesson-figures:end ch-10 -->

### Learning outcomes

After this chapter, you should be able to:

1. Subtract the exact Tuesday pairs.
2. Leave "about 13:50" unsubtracted.
3. Refuse an hour-count from Tuesday afternoon to Wednesday 15:10.

### 10.1 Exact pairs are both written as clock times. "About" is not.

13:48 minus 13:46 is two minutes. That pair is the advert page and then the file. The opening chapters treat a two-minute gap as a click followed by a download, and an hour-long gap as a hint that something else delivered the file. This gap is the short one. The arithmetic uses the two checked clocks only.

14:25 minus 14:20 is five minutes, from the report to the Wi-Fi disconnect. 14:30 minus 14:25 is five minutes, from the disconnect to Ana's permission. 14:30 minus 14:20 is ten minutes for the whole first response. Those three results are the same afternoon, and each endpoint is a time Priya wrote without the word "about".

"About 13:50" cannot be subtracted from 13:48 to produce a precise error. The word "about" is the reason. A note that prints "memory was wrong by two minutes" has invented a precision Tomás did not claim. Wednesday 15:10 is the password change on the next day. The night between Tuesday and Wednesday is not given as a count of hours in this book, so this chapter does not print one.

| Pair | Result this chapter will state |
|---|---|
| 13:46 to 13:48 | 2 minutes |
| 14:20 to 14:25 | 5 minutes |
| 14:25 to 14:30 | 5 minutes |
| about 13:50 against 13:48 | Not an exact difference |
| Tuesday afternoon to Wednesday 15:10 | Not an hour-count in this book |

### 10.2 Worked example

| Offered sentence | Arithmetic | Decision |
|---|---|---|
| "Advert to file is two minutes." | 13:48 minus 13:46 | Keep |
| "Report to permission is ten minutes." | 14:30 minus 14:20 | Keep |
| "Memory missed by exactly two minutes." | "About" blocks the subtraction | Refuse |
| "Fifteen hours passed before the password change." | No such count is printed | Refuse |

> **Synthetic case.** The clocks are the ones already in Priya's notes.

### Errors and limits

Do not turn an approximate memory into an exact error bar.

### Apply

- **EX056 — Which gaps are exact.** Compute the advert-to-file gap and the report-to-permission gap. Is "about 13:50" an exact third gap, and is an hour-count to Wednesday 15:10 printed? *Rubric:* 3 = two minutes and ten minutes; about 13:50 is not an exact gap; no hour-count to Wednesday is printed;; 2 = the two exact gaps and the refused pair;; 1 = one gap;; 0 = none.

<!-- open-cyber-academy:chapter-navigation:start ch-10 -->
### Continue studying — lesson 10 (chapter 10 of 20)

[← Chapter 09](#ch-09) · [Course chapter index](../SUBJECTS.md) · [Chapter 11 →](#ch-11)

Continue to the next authored lesson.
<!-- open-cyber-academy:chapter-navigation:end -->

## Worked answers

<a id="answer-ex056"></a>
**EX056.** Advert to file is two minutes. Report to permission is ten minutes. About 13:50 is not an exact gap. No hour-count from Tuesday to Wednesday 15:10 is printed.