Light from the star betelgeuse takes 640 years to reach earth. how far away is betelgeuse in units of light-years?

The correct answer is - 640 light-years.

The light-year represents the distance the light can pass in one year, so since we have 640 years of light travel to the Earth from the Betelgeuse Star, we have 640 light years of distance.

Now the distance itself is a totally different matter, because the light-year is a distance measure, not a time measure. One light year equals to 9,460,730,472,580.8 km of distance passed by the light in one Julian year, so in 640 years it will come to a distance of 6,054,867,697,356,520 km that the light has passed in order to reach our planet.