QUESTION

CSAT

Medium

Maths

Prelims 2024

On January 1st, 2023, a person saved Rs 11. On January 2nd, 2023, he saved Rs. 22 more than that on the previous day. On January 3rd, 2023, he saved Rs. 22 more than that on the previous day and so on. At the end of which date was his total savings a perfect square as well a perfect cube?

Select an option to attempt

Explanation

The person’s savings increase by Rs. 22 each day. This creates a series of daily savings: 1,3,5,7,...1, 3, 5, 7, ...

Let’s calculate the total savings day by day until we reach a value that is both a perfect square and a perfect cube.

Calculations:

  1. 1st January: Savings = 11 Total savings = 11 (11 is both a perfect square and a perfect cube)

  2. 2nd January: Savings = 33 Total savings = 1+3=41 + 3 = 4 (44 is a perfect square but not a perfect cube)

  3. 3rd January: Savings = 55 Total savings = 1+3+5=91 + 3 + 5 = 9 (99 is a perfect square but not a perfect cube)

  4. 4th January: Savings = 77 Total savings = 1+3+5+7=161 + 3 + 5 + 7 = 16 (1616 is a perfect square but not a perfect cube)

  5. 5th January: Savings = 99 Total savings = 1+3+5+7+9=251 + 3 + 5 + 7 + 9 = 25 (2525 is a perfect square but not a perfect cube)

  6. 6th January: Savings = 1111 Total savings = 1+3+5+7+9+11=361 + 3 + 5 + 7 + 9 + 11 = 36 (3636 is a perfect square but not a perfect cube)

  7. 7th January: Savings = 1313 Total savings = 1+3+5+7+9+11+13=491 + 3 + 5 + 7 + 9 + 11 + 13 = 49 (4949 is a perfect square but not a perfect cube)

  8. 8th January: Savings = 1515 Total savings = 1+3+5+7+9+11+13+15=641 + 3 + 5 + 7 + 9 + 11 + 13 + 15 = 64 (6464 is both a perfect square and a perfect cube, as 64=82=4364 = 8^2 = 4^3)

The total savings amount to 64 on 8th January, 2023, which is both a perfect square and a perfect cube.

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