Calculate the number 843
NUMBERMANIA: Calculate the number 843 using numbers [5, 5, 4, 7, 43, 642] and basic arithmetic operations (+, -, *, /). Each of the numbers can be used only once.Correct answers: 1
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Railroad safetyIn 1870, Thomas S. Hall of Stamford, Conn., received a patent for the first automatic electric block railroad signal system in the U.S. (No. 103,875). When a train entered a block of track, Hall's electromagnetic device automatically set a signal when the locomotive struck a lever fastened to the rail. The signal was set to danger until the train cleared the block. After Hall invented the device in 1867, it was tried out on the New York and Harlem Railroad. |