The codes which when turned upside down gives a number i.e.,
01-10
06-90
08-80
09-60
16-91
18-81
19-61
66-99
68-89
86-98
->so from 0-99 there are 20 such numbers which can create confusion
ex:an intelligence agency decides on a2-digits selected from 0, 1, ....9 so that 1st digit is non-zero.The code, handwritten on a slip, can create confusion when read upside down.How many codes are there for which there is no confusion.
a.80 b. 63
c.71 d.69
sol:The 2-digit number is selected from 10-99(90 codes)
out of these remove 9 codes viz 11,22,33,44,55,66,77,88,99
remaining 81
the codes which can create confusion are 16-91,18-81,19-61,66-99(we have already removed it),68-89,86-98 total 10
therefore ans is 81-10=71.