The average distance from the Sun to the Earth is 1.5 × 10^11 meters and the speed of light is 3 × 10^8. If distance = rate times time, how many seconds on average does it take for the Sun's light to reach Earth?

The problem's question is: "how many seconds does it take..." That's "time".  How are time, speed and distance related?

d = r x t  distance is equal to rate of speed multiplied by time.  Thus t = d ÷ r  This is the relationship we'll use to solve this problem.  Now, let's organize our data:

r = 300,000,000 m/s  Let's write this using scientific notation:

r = 3 x 108 m/s

d = 1.5 x 1011 m

t = d ÷ r  ⇒  t = 1.5 x 1011 ÷ 3 x 108  or:

t = .5 x 103