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Week Number worksheet

Grades 4–5

Solve each problem and show your work. Round answers to 2 decimal places where needed.

1.
Problem 1.
- What do you want to find?: Dates in a week number
- Week numbering: ISO 8601 (Monday start)
- Year: 2,800
- Week number: 11
Find: Week 11 of 2800
Answer
2.
Problem 2.
- What do you want to find?: Week number of a date
- Week numbering: ISO 8601 (Monday start)
- Date: Apr 1, 2025
Find: ISO week number
Answer
3.
Problem 3.
- What do you want to find?: Week number of a date
- Week numbering: ISO 8601 (Monday start)
- Date: Apr 14, 2025
Find: ISO week number
Answer
4.
Problem 4.
- What do you want to find?: Week number of a date
- Week numbering: ISO 8601 (Monday start)
- Date: Oct 23, 2025
Find: ISO week number
Answer
5.
Problem 5.
- What do you want to find?: Dates in a week number
- Week numbering: ISO 8601 (Monday start)
- Year: 1,500
- Week number: 9
Find: Week 9 of 1500
Answer
6.
Problem 6.
- What do you want to find?: Dates in a week number
- Week numbering: US (Sunday start, week 1 has Jan 1)
- Year: 2,000
- Week number: 12
Find: Week 12 of 2000
Answer
7.
Problem 7.
- What do you want to find?: Week number of a date
- Week numbering: ISO 8601 (Monday start)
- Date: Jun 28, 2025
Find: ISO week number
Answer
8.
Problem 8.
- What do you want to find?: Week number of a date
- Week numbering: US (Sunday start, week 1 has Jan 1)
- Date: Mar 17, 2026
Find: US week number
Answer
9.
Problem 9.
- What do you want to find?: Week number of a date
- Week numbering: ISO 8601 (Monday start)
- Date: Jan 19, 2025
Find: ISO week number
Answer
10.
Problem 10.
- What do you want to find?: Dates in a week number
- Week numbering: US (Sunday start, week 1 has Jan 1)
- Year: 2,400
- Week number: 12
Find: Week 12 of 2400
Answer

Answer key: Week Number worksheet

1. Week 11 of 2800: Mon, Mar 13, 2800 – Sun, Mar 19, 2800
1. ISO week 1 is the week containing January 4. January 4, 2800 is a Tuesday, so week 1 starts on the Monday on or before it
  $week 1 starts$ $= Mon, Jan 3, 2800$
2. Move forward 10 whole weeks to reach week 11
  $7 \times 11 - 1$ $= 70 days$
3. The week runs Monday to Sunday
  $week 11$ $= Mon, Mar 13, 2800 – Sun, Mar 19, 2800$
2. ISO week number: 14
1. ISO weeks run Monday to Sunday and belong to the year of their Thursday. This week's Thursday is Apr 3, 2025
  $Thursday's day of the year$ $= 93$
2. Count the whole weeks before that Thursday in its year, then add 1
  $⌊ \frac{93 - 1}{7} ⌋ + 1$ $= 14$
3. ISO week number: 16
1. ISO weeks run Monday to Sunday and belong to the year of their Thursday. This week's Thursday is Apr 17, 2025
  $Thursday's day of the year$ $= 107$
2. Count the whole weeks before that Thursday in its year, then add 1
  $⌊ \frac{107 - 1}{7} ⌋ + 1$ $= 16$
4. ISO week number: 43
1. ISO weeks run Monday to Sunday and belong to the year of their Thursday. This week's Thursday is Oct 23, 2025
  $Thursday's day of the year$ $= 296$
2. Count the whole weeks before that Thursday in its year, then add 1
  $⌊ \frac{296 - 1}{7} ⌋ + 1$ $= 43$
5. Week 9 of 1500: Mon, Feb 26, 1500 – Sun, Mar 4, 1500
1. ISO week 1 is the week containing January 4. January 4, 1500 is a Thursday, so week 1 starts on the Monday on or before it
  $week 1 starts$ $= Mon, Jan 1, 1500$
2. Move forward 8 whole weeks to reach week 9
  $7 \times 9 - 1$ $= 56 days$
3. The week runs Monday to Sunday
  $week 9$ $= Mon, Feb 26, 1500 – Sun, Mar 4, 1500$
6. Week 12 of 2000: Sun, Mar 12, 2000 – Sat, Mar 18, 2000
1. US week 1 is the Sunday-to-Saturday week containing January 1, 2000, which is a Saturday
  $week 1 starts$ $= Sun, Dec 26, 1999$
2. Move forward 11 whole weeks to reach week 12
  $7 \times 12 - 1$ $= 77 days$
3. The week runs Sunday to Saturday
  $week 12$ $= Sun, Mar 12, 2000 – Sat, Mar 18, 2000$
7. ISO week number: 26
1. ISO weeks run Monday to Sunday and belong to the year of their Thursday. This week's Thursday is Jun 26, 2025
  $Thursday's day of the year$ $= 177$
2. Count the whole weeks before that Thursday in its year, then add 1
  $⌊ \frac{177 - 1}{7} ⌋ + 1$ $= 26$
8. US week number: 12
1. January 1, 2026 is a Thursday, which is 4 days after Sunday; Mar 17, 2026 is day 76 of the year
  $weekday of Jan 1, day of year$ $= 4, 76$
2. Add those days so week 1 starts on a Sunday, count whole weeks, then add 1
  $⌊ \frac{76 + 4 - 1}{7} ⌋ + 1$ $= 12$
9. ISO week number: 3
1. ISO weeks run Monday to Sunday and belong to the year of their Thursday. This week's Thursday is Jan 16, 2025
  $Thursday's day of the year$ $= 16$
2. Count the whole weeks before that Thursday in its year, then add 1
  $⌊ \frac{16 - 1}{7} ⌋ + 1$ $= 3$
10. Week 12 of 2400: Sun, Mar 12, 2400 – Sat, Mar 18, 2400
1. US week 1 is the Sunday-to-Saturday week containing January 1, 2400, which is a Saturday
  $week 1 starts$ $= Sun, Dec 26, 2399$
2. Move forward 11 whole weeks to reach week 12
  $7 \times 12 - 1$ $= 77 days$
3. The week runs Sunday to Saturday
  $week 12$ $= Sun, Mar 12, 2400 – Sat, Mar 18, 2400$