Problem
Find a future date using days per month
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.
Measurement & data
Your answer
Watch out Don't assume April also has 31 days like May — that extra day shifts the whole count off by one. April only goes up to the 30th.
How to solve
Strategy Identify Subproblems — Split the 50 days into 'days left in April' and 'days spilling into May.' Subtracting the April part from 50 leaves the days that carry into May, keeping the month boundary clear.
1STEP 1
Count the days left in April
From April 10 to April 30, 20 days remain.
30 − 10 = 20
The distance from April 10 to April's last day equals April's total 30 days minus the 10 already passed — subtracting the part already used from the whole leaves exactly what's left.
Identify Subproblems1.MD.B.3Time2STEP 2
Find how many days spill into May
Of the 50 days, 20 are used in April, leaving 30 days for May.
50 − 20 = 30
The whole 50 days splits — with no day skipped and none counted twice — into the 20 that finish April and the rest that carries into May, so subtracting 20 leaves exactly what spills into May.
Identify Subproblems1.MD.B.3TimeThe 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 forward into May
Counting 30 more days from April 30 lands on May 30.
April 30 + 30 days = May 30
The day after April 30 is May 1, so counting 30 more days from there lands on May 30 — and since May has 31 days, day 30 still falls inside May.
Work Backwards1.MD.B.3TimeAnswer
May 30
April 10 + 50 days = May 30
Check — 20 days from April 10 to April 30, plus 30 more days to May 30, totals 20+30=50 days, and May 30 fits within May's 31 days, so May 30 is correct.
Takeaway
Use up the days left in the current month first, then count the rest into the next month to find the future date.
- Days left in April → 20 days
- Days spilling into May → 30 days
- Counting into May lands on → May 30
Where next?
Level upsuggested
Another one like thissuggested
Same look, different logicsuggested
Step back and solidifysuggested