Machines make work easier by

Groups of organs that work together to complete a process in the body are called: a. cells b. tissues c. nerves d. systems

What is the mass of the same rock on the earth

Light from the sun reaches the earth in 8.3 minutes. the velocity is 3.00 x 10^8 m/s. how far is earth from the sun? i know how to get to d = (3 x 10^8 m/s)(498 sec). i just don't know how to get from there to the answer being 1.5 x 10^11 m.