When 40% of a number is subtracted from 65% of the same number, the result is 84. Half of that number is:

65% − 40% = 25% of the number, so $0.25x=84$ → $x=336$, and half of it is 168. (336 is the number itself — the question asks for half of it — and dividing 84 by 0.40 gives 210, which uses the wrong one of the two percentages.)