IPMAT/Modern Math•25d
xlrijamshedpur

How to solve this?

I did not understand what the question wants to say moreover they say that 2,5 dont create confusion but if you flip 2 upside down it looks like 5 and if you flip 5 upside down it looks like 2 so how is it excluded

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Comments (4)

J
jellyionk
IIM Ranchi
MOD•25d

tried writing them, it depends on the font!

taking the 'default' Arial or Calibri, they don't look that similar.

ask yourself, "would i confuse the two?"

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X
xlrijamshedpur
OP•24d

My handwriting is a bit weird so i thought they look the same!

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X
xlrijamshedpur
OP•24d

Also how do I solve the rest of the question.

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B
basicallybhavesh
IIM Ranchi (IR 2)
MOD•24d

"1, 6, 8, 9, and 0 (only for 2nd digit)"

These are the digits that still look like digits upside down. 0 can only be the second digit because the first digit can't be zero. That's why it says "4 digits" for the first place: 1, 6, 8, 9.

"4 × 4 = 16"

  • First digit: 1, 6, 8, 9 → 4 choices
  • Second digit: any of the 5 digits {0, 1, 6, 8, 9} except the one already used → 4 choices

"69 and 96, 10, 60, 80, 90 will not create confusion"

  • 69 and 96: upside down they still read the same, so there's nothing to confuse.
  • 10, 60, 80, 90: upside down they'd start with 0 (like "01"), which isn't a valid code, so nobody would mistake them for another code.

That removes 6 codes: 16 − 6 = 10 confusing codes.

Final step: 81 − 10 = 71 codes with no confusion.

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