There are some cards. On each card 2-digit code is written selected from 0, 1, 2………9. But the card on which the code is written allows confusion between top and bottom, because these are indistinguishable. Thus, for example, the code 81 could be confused with 18. How many codes are there such that there is no possibility of any confusion?
Explanation:
Total number of 2-digit codes = 10 × 10 = 100
Digits which can be read upside down are 0, 1, 6, 8 and 9.
2-digit code formed using these digits that can be read upside down.
Number of 2-digit codes that can be formed using these 5 digits = 5 × 5 = 25.
Out these 25 codes there are some codes which will actually not create confusion: 00, 11, 88 , 69 and 96
Hence, (25 – 5 =) 20 codes can create confusion.
∴ Codes which will not create confusion = 100 – 20 = 80.
Hence, option (c).
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