You just need to multiply the total mass by the decimal value of the part that is tin. 133.8*0.103=13.8g (following the rules of significant figures).
Density is an intrinsic property, so it is independent of the amount of substance present: one gold coin would have the same density as a solid gold boulder.
So if the density of gold is 19.3 g/cm³, the density of a bar of gold and the pieces into which the bar is cut would all be 19.3 g/cm³.
When the amount of gas in a container is increased, the volume increases. Lussac's law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.
The answer is b. The sum of the individual gas pressure in the mixture