Trouble finding equation w/ % comp and isotopes with % abundance?

I have two questions I am having difficulty answering. The first states "a compound is made up of carbon, hydrogen, and oxygen atoms. It has 52.14% C and 34.73% O. What is the simplest formula of the compound?" I can figure the compound contains 13.13% H but I don't know where to go from there. The next question says "Magnesium has three isotopes (Mg-24, Mg-25, and Mg-26). Complete the m spectrum given below." Only the abundance of Mg-25 is given and it is 10%. Please answer these questions to the best of your ability and explain how you got your answer or show work. Thanks ahead of time. I have received some great help here in the past.