Rate of fuel consumption $f_c$ (litres per hour) of a truck varies with truck speed $x$, (kmph) as given below

$f_c = 20 + \frac{x^2}{50}$

The fuel price is Rs. 70 per litre. Other costs amount to Rs. 500 per hour. If the truck travels 100 km from a coal mine to a thermal plant, the speed of the truck, in kmph, that minimizes the total cost is _____.