Sensim Math · Depth 한국어

2-2 · Time

Find a future date using days per month

1.MD.B.3 · adapt · grade 1

Archetype: Elapsed Time and Base-Sixty Regrouping · step in a 10-type progression

▶ Practice — 8 problems

April has 30 days and May has 31 days. Starting from April 10, 2024, what is the date 50 days later? Give the month and day.

Jan Mar May Jul Aug Apr Jun Sep Nov Knuckles (raised): 31 days Dips (between): 30 days
Show solution

Understand

April has 30 days and May has 31 days. Starting from April 10, 2024, find the date that is 50 days later (give month and day).

Givens
  • April has 30 days.
  • May has 31 days.
  • Start date is April 10, 2024.
  • We move forward 50 days.
  • The knuckle mnemonic in the figure confirms April is a 30-day month and May is a 31-day month.
Unknowns
  • The month and day that is 50 days after April 10, 2024.
Constraints
  • Days roll over into the next month once a month's days run out.

Plan

#7 Identify Subproblems · also uses: #11 Work Backwards

Break the 50 days into 'days left in April' plus 'days spilling into May.' Subtract the April part from 50, and the rest counts into May. This keeps the month boundary clear.

Execute

#7 Identify Subproblems 1.MD.B.3
From April 10 to the end of April (April 30) is 30 minus 10 days.
30−10=2030 - 10 = 20
April 10 plus 20 days lands exactly on April 30, the last day of the month.
#7 Identify Subproblems 1.MD.B.3
Of the 50 days, 20 are used getting to April 30. The remaining days fall in May.
50−20=3050 - 20 = 30
After landing on the last day of April, the leftover 30 days are counted into the next month.
#11 Work Backwards 1.MD.B.3
Starting from April 30, 30 more days lands on May 30 (since May has 31 days, day 30 is still within May).
April 30+30 days=May 30\text{April } 30 + 30 \text{ days} = \text{May } 30
April 30 + 1 day is May 1, so +30 days is May 30, which fits inside May's 31 days.
Answer: May 30

Review

Check the total span: April 10 to May 30. April part is 20 days (to April 30) and May part is 30 days (to May 30), totaling 20 + 30 = 50 days. The result May 30 is within May's 31 days, so it is valid.

Count by whole months: April 10 + 20 days = April 30, then add 30 more days into May (which has 31 days) to reach May 30 — same answer.

Standards · min grade 1

  • 1.MD.B.3 Tell and write time in hours and half-hours using analog and digital clocks — Working with calendar dates and days within months to find a future date.
💡 Use up the days left this month first, then count the rest into the next month: simple Grade 1 calendar counting!