Find the third proportion of X and 30, when 45:12::75:x
45
First solve 45:12::75:x → x = (75×12)/45 = 20. The third proportional of two numbers a and b is b²/a, so for 20 and 30 it is 30²/20 = 900/20 = 45, matching option (B), which is also the option marked in the answer sheet.
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