So I tried and tried this problem but I couldn't ever find the correct density. I know that you have to convert the inches to centimeters and then you have to use the formula for the volume of a sphere to find the volume. But somehow I'm not getting the correct answer so any help will be GREATLY appreciated. Thank you 
A solid metal sphere with a diameter of 0.75 inches weighs 40.9 grams. What type of metal is the sphere? The densities of common metals are given in the list of possible answers.
Answer
aluminum (density = 2.70 g/cm3)
iron (density = 7.87 g/cm3)
lead (density = 11.3 g/cm3)
tungsten (density = 19.3 g/cm3)
platinum (density = 21.5 g/cm3)
titanium (density = 4.51 g/cm3)
nickel (density = 8.90 g/cm3)
vanadium (density = 6.0 g/cm3)
A solid metal sphere with a diameter of 0.75 inches weighs 40.9 grams. What type of metal is the sphere? The densities of common metals are given in the list of possible answers.
Answer
aluminum (density = 2.70 g/cm3)
iron (density = 7.87 g/cm3)
lead (density = 11.3 g/cm3)
tungsten (density = 19.3 g/cm3)
platinum (density = 21.5 g/cm3)
titanium (density = 4.51 g/cm3)
nickel (density = 8.90 g/cm3)
vanadium (density = 6.0 g/cm3)