The rate of people getting in should equate the rate of people coming out. If a meal takes one hour, in that time 40 tables would be emptied, so on average a couple would enter every 90 seconds.
As you came in a random moment, the first couple will have to wait, on average, 45 seconds. The next 17 couples will wait 90 seconds each, and you'll have another 90 seconds to wait -- all together, 27 minutes and 45 seconds.
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