Hope you understand how to work out those types of questions now xD ;)
A hope it helps ur welcome
Platinum plus chlorine equals 0.889 grams whereas the platinum alone is 0.509grams so 0.509/0.889grams = 57% of the mass of the platinum plus chlorine. The chlorine would be 0.889-0.509=0.38 grams. The chlorine would make up about 42.7% of the total mass in grams.
10, deca means ten a decagon is a ten side polygon, a decimetre is one tenth of a metre a decade is ten years so your answer is ten.