The speed of light in vacuum is taken as unity. If light takes 6...

Speed of light in vacuum, $c = 1$ unit/s.

Time taken by light to reach the Earth from the Sun, $t = 6$ min $40$ s.

Converting time into seconds:

$t = 6 \times 60 + 40 = 360 + 40 = 400$ s.

Distance between the Sun and the Earth is given by:

$d = c \times t$

$d = 1 \times 400 = 400$ new units.

Answer: $400$