First of all, the speed of light is 3 x 108 meters per second, not 3 x 105.
It will take 10 milliseconds for a ray of light to travel 3 x 106 meters at a speed of 3 x 108 meters per second.
If the number in the question were correct, which they are not, the answer would be ten seconds.
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One light year, or the distance that a beam of light can travel in one year, at 186,000 miles per second.
Now, normally, time is linear and inescapable. However, at extreme speeds, approaching the speed of light, special relativity states that time is perceived as slower by the people travelling at that speed. One example of this is the premise of Planet of The Apes. The astronauts are travelling at something near the speed of light, so while a short time passes for them, a very long time passes for Earth.
Any number that doesn't change is a constant. Some famous ones are the gravitational constant, the speed of light in a vacuum, and Planck's constant.
mc2 could mean many things, but I assume you are talking about Einstein's famous equation E=mc2. I won't even pretend to know all the information behind this equation, but my understanding is that it basically calculates the amount of energy that could be spontaneously created by any mass. In the equation, E=mc2 E = Energy M = Mass C = Speed of Light (or 299,792,458 meters/second)
C equals the speed of light in a vacuum, which is approximately 299,792,458 meters per second. So if you're looking for a simple answer, there you have it. Just remember, I'm not here to hold your hand through the world of physics, honey.