MCQMediumJEE 2024Arithmetic Progression (AP)

JEE Mathematics 2024 Question with Solution

A software company sets up mm number of computer systems to finish an assignment in 1717 days. If 44 computer systems crashed on the start of the second day, 44 more crashed on the third day, and so on, taking 88 more days to finish, then the value of mm is equal to:

  • A

    125125

  • B

    150150

  • C

    180180

  • D

    160160

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The company initially planned to use mm computer systems for 1717 days. After the first day, 44 systems crashed each day, and the work took 88 extra days, so the total time became 2525 days.

Find: The value of mm.

Without any crash, total work required is

17m17m

computer-days.

With crashes, the work done day-wise forms an arithmetic progression:

m+(m4)+(m8)++(m4×24)m + (m-4) + (m-8) + \cdots + (m-4\times 24)

because there are 2525 days in total.

Equating actual work to required work:

17m=m+(m4)+(m4×2)++(m4×24)17m = m + (m-4) + (m-4\times 2) + \dots + (m-4\times 24)

so

17m=25m4(1+2++24)17m = 25m - 4(1+2+\dots+24)

Now,

1+2++24=24×252=3001+2+\dots+24 = \frac{24\times 25}{2} = 300

Substituting,

17m=25m4×30017m = 25m - 4\times 300 25m17m=120025m - 17m = 1200 8m=12008m = 1200 m=150m = 150

Therefore, the value of mm is 150150. The correct option is B.

Direct AP Setup

Given: Planned work is 17m17m computer-days, but due to daily crashes of 44 systems from day 22 onward, the job finishes in 2525 days.

Find: The value of mm.

Treat the actual daily working systems as an AP with first term mm, common difference 4-4, and 2525 terms.

Using the compact form from the extracted working:

17m=25m4(1+2++24)17m = 25m - 4(1+2+\dots+24) 8m=4×24×2528m = 4\times \frac{24\times 25}{2} 8m=12008m = 1200 m=150m = 150

Therefore, the correct option is B.

Common mistakes

  • Taking the number of lost-system terms as 2525 instead of 2424 is incorrect, because no system crashes on the first day. Start the reduction from day 22, so the deductions are 0,4,8,,4×240,4,8,\dots,4\times 24.

  • Using 1717 terms in the AP for actual work is wrong, because the assignment finished in 2525 days after taking 88 extra days. The actual work sum must be over 2525 days.

  • Equating the AP sum to 25m25m directly is incorrect. 25m25m is only the total if no crashes occurred during those 2525 days. You must subtract the lost work caused by the crashed systems each day.

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