According to the 1998 Information Please Almanac, the official speed record for running one mile was held by Richard Webster of England in 1865, with a time of 4 minutes 36.5 seconds.

Roger Bannister, also of England, was the first to run a mile in less than four minutes, clocking a time of 3 minutes 59.4 seconds in 1954.

There are (of course) 60 seconds in each minute. Therefore, we could convert both times into seconds, writing them as follows:
a1865 = 276.5 seconds and a1954 = 239.4 seconds.
Assuming the speeds decrease from year to year in an arithmetic pattern, find d.

Question

According to the 1998 Information Please Almanac, the official speed record for running one mile was held by Richard Webster of England in 1865, with a time of 4 minutes 36.5 seconds.

 
Roger Bannister, also of England, was the first to run a mile in less than four minutes, clocking a time of 3 minutes 59.4 seconds in 1954.

There are (of course) 60 seconds in each minute. Therefore, we could convert both times into seconds, writing them as follows:
a1865 = 276.5 seconds and a1954 = 239.4 seconds.
Assuming the speeds decrease from year to year in an arithmetic pattern, find d.

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