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Bob can finish a job in 4040 days, if he works alone. Alex is twice as fast as Bob and thrice as fast as Cole in the same job. Suppose Alex and Bob work together on the first day, Bob and Cole work together on the second day, Cole and Alex work together on the third day, and then, they continue the work by repeating this three- day roster, with Alex and Bob working together on the fourth day, and so on. Then, the total number of days Alex would have worked when the job gets finished, is

Entered answer:

Solution

✅ Correct Answer: 11

When we say someone can finish a job in a certain number of days, we're talking about their work rate.

Given information:

  • Bob finishes the job in 40 days
  • Alex is twice as fast as Bob → Alex finishes in 20 days
  • Alex is thrice as fast as Cole → Cole finishes in 60 days

If Alex is twice as fast as Bob, he takes half the time. If Alex is thrice as fast as Cole, Cole takes three times as long as Alex.


Instead of working with fractions, let's assume the total work = 120 units.

We choose 120 because it's the LCM of 40, 20, and 60. This makes our calculations much cleaner!

Now we can find each person's daily work rate:

  • Alex's rate = 120 ÷ 20 = 6 units/day
  • Bob's rate = 120 ÷ 40 = 3 units/day
  • Cole's rate = 120 ÷ 60 = 2 units/day

The pattern repeats every 3 days:

  • Day 1: Alex + Bob work together
  • Day 2: Bob + Cole work together
  • Day 3: Cole + Alex work together

Work done each day:

  • Day 1: Alex + Bob = 6 + 3 = 9 units
  • Day 2: Bob + Cole = 3 + 2 = 5 units
  • Day 3: Cole + Alex = 2 + 6 = 8 units

Total work per 3-day cycle = 9 + 5 + 8 = 22 units


We need to complete 120 units of work.

How many complete 3-day cycles?

120 ÷ 22 = 5 remainder 10

This means:

  • 5 complete cycles = 15 days
  • Work done in 15 days = 5 × 22 = 110 units
  • Remaining work = 120 - 110 = 10 units

What happens next?

  • Day 16: Alex + Bob work → 9 units completed
  • Total after Day 16: 110 + 9 = 119 units
  • Still need: 120 - 119 = 1 unit
  • Day 17: Bob + Cole work at 5 units/day
  • Time to finish 1 unit = 15\tfrac{1}{5} day

Total time to finish = 16 + 15\tfrac{1}{5} = 16.2 days


In the first 15 days (5 complete cycles):

  • Alex works on Day 1 and Day 3 of each cycle
  • That's 2 days per cycle × 5 cycles = 10 days

On Day 16:

  • Alex works with Bob for the full day = 1 day

On Day 17:

  • Only Bob and Cole work (Alex rests) = 0 days

Total days Alex worked = 10 + 1 + 0 = 11 days


In work rate problems, always:

  1. Convert rates to a common unit (using LCM helps!)
  2. Identify the repeating pattern
  3. Calculate complete cycles first, then handle the remainder
  4. Count carefully, especially when the job finishes mid-cycle

Answer: 11

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