Given that x> 50, the gas mileage is 10+ .15(x- 50)= 10+ .15x- 7.5= 2.5- .15x m/g. With gas at $3 /gal, that would cost 3/(2.5- .15x) dollars per mile and so the gas cost for 70 iles will be 210/(2.5- .15x) dollars. It will take 70/x hours to drive 70 miles at x miles per hour so the truck driver wil have to be paid 210/x dollars. The problem, then, is to minimize 210/(2.5- .15x)+ 210/x. You can factor out the 210 and just ignore it. What value of x will minimize 1/(2.5- .15x)+ 1/x?

Take the derivative with respect to x and set it equal to 0.