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Mathematical sampling example
(1) Because there is no specific data, it is impossible to judge the mode.

(2) The frequency ratio of each group is 3: 4: 9: 8: 6: 2 from left to right,

∴ If the frequency of 1 group is 3a, the frequencies of other groups are 4a, 9a, 8a, 6a, 2a,

The frequency of the third group is 9,

∴9a=9,

∴a= 1,

∴ The number of students selected in this survey is 3a+4a+9a+8a+6a+2a = 32a = 32;

(3) According to (2), the number of people with 90 points or above is 8+6+2= 16.

∴ The proportion of students with 90 points or above in mathematics is 1632× 100% = 50%.