Question 1136050
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Moving 3.5 kilometers in 2 minutes, taxi driver has the speed  {{{3.5/((2/60))}}} = {{{(3.5*60)/2}}} = 3.5*30 = 105 km/h, 

which is an obvious violation of the 50 km/h speed limit.



Moving 3.5 kilometers in 4 minutes, taxi driver has the speed  {{{3.5/((4/60))}}} = {{{(3.5*60)/4}}} = 3.5*15 = 52.5 km/h, 

which is a violation of the 50 km/h speed limit, too.




Only moving 3.5 kilometers in 6 minutes, taxi driver reaches his goal without violation of the speed limit.

His speed in this case is  {{{3.5/((6/60))}}}  {{{(3.5*60)/6}}} = 3.5*10 = 35 km/h.



<U>ANSWER</U>.  His safe highest speed is 35 kilometers per hour to reach his goal.
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Ignore other answer by @rothauserc, since it is INCORRECT SOLUTION.