How would you build an engine from scratch? Can you estimate the revenue from 2012 Olympic ticket sales? How many people are watching YouTube in a given hour?

These are just a few of the tough questions job seekers were asked by interviewers in the past year, according to a report released Friday by Glassdoor.com. Using rankings and reviews of interviews posted by thousands of users over the past 12 months, the career website listed the 25 companies that put job candidates through the most rigorous hiring process. (See the full list below).

You'll likely have to survive multiple interviews and a variety of tests and questions to snag a job at one of these firms. While the average job seeker faces a 16-day interview process, candidates for most of the companies on the list reported interview times that spanned weeks or even months, according to Glassdoor.

McKinsey & Co., a management consulting firm that advises many of the world's largest companies, was the toughest interviewer for the third year in a row, according to Glassdoor. Candidates reported up to 9 rounds of interviews, with tests ranging from case studies to a math exam that one candidate compared to the dreaded GMAT.

"The process itself was grueling - training for case studies in parallel to a full-time executive job is not easy - but also very motivating," one candidate wrote on Glassdoor.

Those who get hired are rewarded handsomely. The average base salary for a McKinsey associate is $127,000 a year, according to Glassdoor. McKinsey declined to comment.

Three other consulting firms rounded out the top 10, including Boston Consulting Group and Bain & Co. Technology firms, such as Facebook (FB), Google (GOOG) and Microsoft (MSFT), also dominated the list, filling 11 of the top 25 spots.

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Quick! How much revenue did the 2012 Olympics rake in from ticket sales? Such "market sizing" questions are aimed at gauging a candidate's comfort with numbers and their analytic process, said consulting firm Bain.

"Whether it's X number or Y number, it's less important to me than how you went about answering that question," said Keith Bevans, head of Bain's consultant recruiting team.

How about: How many people watched YouTube in the last hour? While the question was reportedly asked of a prospective Google search analyst in Singapore, a company spokesperson said it has moved away from brainteasers and instead prefers questions aimed at measuring cognitive ability, leadership, job-specific skills and the candidates overall "Googley-ness."

Some company's questions are more personal, such as "What do you do when there is no answer?" (Microsoft) and "What kind of people do you dislike the most?" (Stryker, a medical equipment company). "While we are trying to assess their technical skills, we are aware that candidates are also 'interviewing us' as a potential employer," Nicole Dresser, a staffing consultant for Microsoft's Silicon Valley office, said in a statement.

Some questions are just plain unusual, such as software firm ThoughtWorks' request for a prospective software engineer to "tell us a story [in] which the title is Green Hat."

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Other employers that made Glassdoor's list included investment firm BlackRock (BLK), Procter & Gamble (PG) and Teach for America, the nonprofit that enlists recent graduates to teach in low-income schools.

More than 50,000 applicants vied for less than 10,000 spots in Teach for America's teaching corps last year. To find the best candidates, the nonprofit employs a rigorous four-step interview process that culminates in a daylong event, including individual and group interviews and a sample teaching lesson, to really get to know candidates.

As a result, Teach for America had the longest interview process out of all the companies, with an average of 55 days, according to Glassdoor. "There is a lot we have to learn," said Sean Waldheim, Teach for America's vice president of admissions. "There aren't shortcuts."

Despite the rigor, many job seekers didn't leave feeling discouraged. In fact, the majority of candidates for nearly all of the companies on the list reported a "positive" interview process.

"Just because it is a tough interview, that doesn't mean it's a bad interview," said Scott Dobroski, a Glassdoor spokesman.