Neptune’s average distance from the Sun is 4.503 * 10^9 km. Mercury’s average distance from the Sun is 5.791 * 10^7 km. About how many times farther from the Sun is Neptune than Mercury?

77.75 times. You may round it up to 78 times.
you simply divide the greater distance by the smaller distance. So you get 4.503 x 10^9 / 5.791 x 10^7 = 77.758.