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dec2023


What is the fastest way to identify the month must start on Sun, Mon, Tues? I am able to find if I test all cases Mon-Sun, but it takes too long
dec2023

Looking at your doubt, I can see you understand the concept but need a more efficient approach. Here's the fastest method to solve this type of problem:

The Key Pattern Recognition:
For any 31-day month, remember: \(31 = 28 + 3 = 4 \times 7 + 3\)

This means May has exactly 4 complete weeks plus 3 extra days. These 3 extra days are critical – they're the only days that appear 5 times instead of 4.

The Quick Rule: In a 31-day month, the first 3 consecutive days of the month will appear 5 times. That's it!

Application to Your Problem: For May to have 5 Tuesdays, Tuesday must be among the first 3 days:
  • If May starts on Sunday → Days 1, 2, 3 are Sun, Mon, Tue
  • If May starts on Monday → Days 1, 2, 3 are Mon, Tue, Wed ✓
  • If May starts on Tuesday → Days 1, 2, 3 are Tue, Wed, Thu ✓
  • If May starts on Wednesday or later → Tuesday is NOT in the first 3 days ✗

Result:
3 favorable outcomes out of 7 possible starting days
Probability: \(\frac{3}{7}\)

Time-Saving Strategy for Test Day:
  • Identify extra days: \(\text{Month days} - 28\)
  • Those extra consecutive days (starting from day 1) appear 5 times
  • Count how many starting days include your target day in this range

Quick Check Method:

For 30-day months → 2 extra days → smaller probability window
For 31-day months → 3 extra days → larger probability window

This pattern works for any day of the week in any 31-day month, cutting your solution time from testing 7 cases to instantly identifying the 3 favorable cases.

Answer: D \(\left(\frac{3}{7}\right)\)
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