A word problem

[nonEnglish]

"Robert can swim the first 50 meters of a race in 1 minute. Then he slows down by 12 seconds for each of the next 50 meters of a race. How long will it take Robert to swim a 400-meter race?"

It takes Robert 72 seconds to finish the second 50 meters; but, according to the wording of the question, does it take him 72 or 84 seconds the third 50 meters ?

Thank you.

[Temico]

Then he slows down by 12 seconds for each of the next 50 meters of a race.

The key word is by which means that to swim each 50-meter section of the race takes longer than the previous 50-meter section by 12 seconds. i.e. Section 1 (60secs.), Sec. 2 (72 secs), Sec. 3 ( 84secs.)...........

Without the word by, the sentence would read, "Then he slows down 12 seconds for each of the next 50 meters of a race." which means that the slow-down period is the same for all the sections after the initial one.

Another example; "At the auction, the bidding price was raised by $10 each time." ( e.g. If the initial bid was $100, the next consecutive bids would be $110, $120, $130, $140.......)