Election Math Favors de Blasio It is not rocket science. The odds favor Bill de Blasio. With tens of thousands of votes from the Democratic mayoral primary still to be counted, Mr. de Blasio needs only about one in three of them to remain above the 40 percent threshold he passed in the unofficial count to avoid a runoff against the second-place finisher, William C. Thompson Jr., on Oct. 1. The math will not be lost on Mr. Thompson as he mulls whether to remain in the race. Based on the preliminary count from lever voting machines and emergency ballots cast where machines were not working, about 645,000 votes were cast in the election on Tuesday. Mr. de Blasio received 260,000 votes, or about 2,100 more than he needed to surpass 40 percent. The Board of Elections will begin its official canvass of the machines on Friday. On Monday, the board will begin counting the more than 16,000 valid absentee ballots that have been received (they will continue to be accepted until Tuesday as long as they are postmarked by Primary Day eve). The board will also start tallying thousands more affidavit ballots cast by New Yorkers whose names were not in the official voter rolls consulted by poll workers, but who insisted they were nonetheless eligible to vote. Depending on the number of paper ballots and the degree to which they are challenged by either candidate, that process is likely to take several days. The math, though, is relatively simple. If there are 20,000 mail ballots to be counted and another 10,000 or so affidavit ballots, Mr. de Blasio would need, at most, one in three of those votes to remain above 40 percent. That 33 percent or so of the votes would be less than his preliminary total of 40.3 percent. So unless the bulk of the uncounted votes were from a Thompson stronghold - and they were few and far between - Mr. de Blasio would appear to be in a relatively comfortable position as the official count proceeds.