Problem
Find a future date using days per month
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).
Measurement & data
Your answer
Watch out 50 days later means counting forward from April 10, not counting April 10 itself as the first day.
How to solve
Strategy Identify Subproblems — 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.
1STEP 1
Count the days left in April
From April 10 to the end of April (April 30) is 30 minus 10 days.
30 - 10 = 20
April 10 plus 20 days lands exactly on April 30, the last day of the month.
1.MD.B.3Identify Subproblems2STEP 2
Find how many days reach into May
Of the 50 days, 20 are used getting to April 30. The remaining days fall in May.
50 - 20 = 30
After landing on the last day of April, the leftover 30 days are counted into the next month.
1.MD.B.3Identify SubproblemsThe 50 days forward split into the 20 days that finish out April and the 30 days that carry on into May.
Why?
The whole 50-day jump can be cut at the end of April into two back-to-back stretches with no day skipped and no day counted twice, so the two stretches add back up to 50.
🧱Whole is the sum of its partsBreak a thing into pieces with no gaps or overlaps and the pieces add back to the whole.
Why?
The first stretch uses 20 days to reach the end of April, so the second stretch is whatever is left when those 20 are taken away from 50, and taking away is how we find the missing part of a known total.
🧱Inverse operationsSubtraction undoes addition and division undoes multiplication — they reverse each other.
3STEP 3
Count into May
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
April 30 + 1 day is May 1, so +30 days is May 30, which fits inside May's 31 days.
1.MD.B.3Work BackwardsAnswer
May 30
April 30 + 30 days = May 30
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.
Takeaway
Use up the days left this month first, then count the rest into the next month: simple Grade 1 calendar counting!
- Count the days left in April
- Find how many days reach into May
- Count into May
Where next?
Level upsuggested
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